This happened.

Update, Episode 1-2 Synopsis
 
” In the back-to-back season premiere episodes, titled “The Beginning of the End,” Princess Celestia and Princess Luna decide to retire and hand off the ruling of Equestria to Twilight Sparkle and her friends, which sends Twilight into a tailspin of self-doubt. Meanwhile, an ancient villain bands together a legion of nefarious characters to conquer all of Equestria, challenging Twilight Sparkle and the rest of the Mane 6 to step in and save the day.“

Sigh… The proper answer is too big… Here is an abridged version…
 
@Chopsticks  

 
What do you mean?
 
And here is another definition of lampshading: “the writers’ trick of dealing with any element of the story that threatens the audience’s Willing Suspension of Disbelief, whether a very implausible plot development, or a particularly blatant use of a trope, by calling attention to it and simply moving on”. Not even my definition. Sounds a lot worse, and it’s also colloquially known as “bad writing” or “not thinking ahead”, but like I said, I don’t mind that they’re doing this (if they are, we’re speculating on an episode that hasn’t aired yet).  
I’ve probably done it too…
 
The problem is that, depending on how it’s done, and to what, it feels dishonest. Like throwing a temper tantrum, yelling that one didn’t mess up, and that all the times that something shouldn’t have happened, but did, was intentional. Even if in retrospect.
 
Suppose that in the episode the sisters mention how lately they’ve been incapable of dealing with the problems that cropped up and that is why they’re ‘retiring’. Now suppose that the sisters, instead, say that Twilight and friends are ready to take on their job, and that is why they’re retiring, no mention of their incapacity to deal with the problems that the Mane Six fix, or that they aren’t needed anymore.
 
While this addresses the ‘issue’, it does so in a very childish way, because everyone knows that in no moment the production staff stopped writing a particular scene and went “Wait… Celestia is getting old and can’t deal with this anymore. That is why she’s going to leave it to Twilight/Cadance/whoever!” For this to work, for starters, the story would have to be ABOUT HER, and it never was. This is insulting to the audience’s intelligence and sounds like the producers are incapable of admitting a mistake or negligence, for whatever reasons it happened.
 
The alternative isn’t trying to fix a problem that can’t be fixed, and is instead, actually, moving on with Twilight’s story as if nothing happened. This is ‘good lampshading’.
 
The problem is that the Sisters never called their inability to deal with those problems at the appropriate time. When Celestia and Luna ‘defeated’ King Sombra in the past, the cartoon wasn’t telling a story of how the Sisters did their best to beat an evil overlord, it was setting up the stage for Twilight and friends to do it. Not even mentioning the failure that is the the whole ‘test’ subplot. Why were the two even fighting? They still had access to the Elements of Harmony at the time.
 
When Celestia fought Chrysalis, she was fully confident she could defeat her and the cartoon downright said that the reason she didn’t was because Chrysalis was pretty much high on Love.  

 
Celestia is used as a measure of power! The first season treated Nightmare Moon as godlike and made a huge deal of the ability of bringing the day and night. This is why memes like Godlestia and Trollestia exist. People didn’t believe that Celestia was incapable of dealing with stuff and left them to Twilight intentionally. Most of what comes later, as far as Celestia is concerned, is a huge patchwork of writers trying to cover their tracks, starting with Hearth’s Warming Eve, where the writers couldn’t be bothered to think for a couple seconds with finding a role for the unicorns to fulfill, and that was a previous episode, before Celestia lost her fight.
 
Right now, I’m not angry at the cartoon for making Celestia a tired old lady that just wants to rest. I’m not even angry at the fans for swallowing this, or for the cartoon to drag Luna along as if the two are inseparable (which they were supposed to be, but never were). If that is what they want Celestia to be, then fine. I just hope they give a beloved character some dignity in the end. They can’t stop people from writing better fanfiction than their original material anyways…
 
And while I don’t care if in the cartoon Celestia is so powerful that she can decide to release the greatest threat to Equestria on a whim, or as pathetic that she has to bow her head to the EEA, why do I pick on the cartoon for messing up their job? Because I’m not the one that said Celestia rules anything and has characters talking about her like she’s Equestria’s own Jesus Christ before Twilight. As far as I’m concerned, she didn’t need to be anything to do her job of raising the sun. Or for Luna to raise the moon and protect dreams. It’s the cartoon that started something it can’t finish without gutting the characters or infringing on age ratings.
 
So, in the end, what am I saying? That the best way to deal with this is to simply say that Celestia and Luna decided to retire. Why? Who cares? The cartoon is ending.
 
It’s like breaking a window to evacuate people from a building on fire. It’s fine that you broke the window to save lives, but the window is still broken. It gets cold at night, people are going to miss it. How do you fix it? You don’t. You get everyone out of the building and move on. No one is going to care about the broken window after the building is done burning. The important part is that you saved the people.
 
Instead of thinking of Celestia as some old badass that needs all sorts of mental gymnastics to exist in the past, and is now retiring to die forgotten in a hole when all attention went to the next generation, I’d rather take all the poor writing the cartoon gave her and let her end (or even die, if that is the case) in a final blaze of how out of everyone’s league she was. To the sound of Solence’s Legends Never Die. Or perhaps Malukah’s Beauty of Dawn.
 
….. Yeah… Right…

Funny I actually addressed this in a fanfic, the followup I never finished unfortunately.
 
But I think you’re taking it too symbolically deeply with connections to the show’s staff.
 
@moonlightaveger  

 
Lampshading = the show creators give you a reasonable explanation (or) they let you know they understand the issue is there, and it’s a bit of a bewilderment to those in the show as well, by way of the characters. In this example:
 
Luna: “Equestria is enjoying its longest period of harmony, in years.”
 
Celestia: “And it’s all thanks to you and your friends.”
 
Luna and Celestia acknowledged exactly what you said. And there’s dozens of reasonable explanations as to why.
 
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Celestia’s been successfully in power for nearly 1,000 years. We don’t know a lot of what transpired in that time between Luna’s banishment and return. One possibility is it’s an adjustment to go from that to a dozen major problems that weren’t overcome except by your star pupil and secret weapon contingency plan you setup with the Elements.
 
It seems that Luna and Celestia, in their younger years, took care of their problems temporarily in the most straightforward way they knew how, through major sacrifice. Assuming their Alicorn power weakens very slowly over time (or possibly even bleeds back into Equestria after exhaustive use), it would mean that they knew in advance their fixes weren’t going to last forever.
 
Especially if you follow the comics, you can see a lot of this, like when Celestia blasted the entire Changeling swarm with a sunbeam so strong it burned all the holes in them. Or with the Pillars.
 
Discord. King Sombra. The Dazzlings. Pony of Shadows. Nightmare Moon. Queen Chrysalis. Tirek. Heck, even the Crystal Empire blizzard in season 6. Everything was a temporary fix that required immense saturation of power or sacrifice.
 
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So more likely by how things developed, what did Celestia uselessly do this whole time? She set up and devised the Elements of Harmony to be implanted and used by Twilight and Friends, during the time she knew Equestria’s greatest threats would be able to crack through. That’s why Celestia has such stately calm. She’s would’ve been pulling Twilight in front to strengthen her and give her a chance. They gave her the task of saving the Crystal Empire as a test instead of intervening.
 
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As far as what they do day-to-day. Running a kingdom without a court system like a Congress or Parliament puts a heavy administrative load on just 1-2 Princesses and their handful of assistants. Equestria seems to be set up like city-states that sees to their own affairs, but very limited influence in the wider government. There’s no army of Equestria; just the Royal Guard directly loyal to the Princesses.
 
Times have obviously changed since Equestria’s been threatened rapidly, suddenly. I’m sure Princess Celestia and Luna realize their time and requirement of need is no longer what it used to be.

@Chopsticks  
Does calling it lampshading make it any more reasonable? The way the cartoon has been treating Celestia, she might as well not exist, or the cartoon might never had mentioned her as ruler or anything of importance other than having this ability to raise the sun. This whole mess that Celestia is wouldn’t exist if that was the case. That is, assuming that the episode’s argument is that things go wrong because Celestia and Luna ‘retired’.
 
The problem is that the cartoon never called them on their incompetence for this to be a thing. It was never aware, and it was always a product of the cartoon’s writers not caring about that. We have an episode where Celestia was imprisoned in the moon for years, and it’s not even a point of discussion. They didn’t even bother to show that the sun was missed after years of night.
 
So NOW they want to pull this? The only reason is that this is the last season.
 
Please don’t get me wrong, if that is what they want to do, I’m fine with it. At least the cartoon would be trying to come to terms with what it’s been doing with Celestia, and trying to fix her, specifically, because Luna never felt incompetent at her job, after the cartoon decided that she can enter dreams and guards them from nightmares. Hell, I don’t even know what is it that Celestia actually does because the episode that was supposed to show what her job was like made a joke of everything and never called Luna on her lack of skill at Celestia’s job.
 
Which brings me to another point: why is Luna ‘retiring’ too? If it is about coming back and saving the day, then all Luna is going to do is get in the way, because she doesn’t need this treatment and Celestia still won’t be able to show what made her important, because one of those plots about the world ending is going to get in the way. All that will happen is that the sisters will do something that the Mane Six used to do, while they can’t do it, and in the end, Celestia is still worth jack because the cartoon won’t let her be useful on her own, on her terms.

@moonlightaveger  

 
I think they lampshade that though. They said Equestria has experienced peace for several years, because Twilight and friends were there. They were admitting they haven’t done the best job, and that they’re pretty much not really needed anymore (remains to be seen).
 
 
For all we know, it could be an episode where they struggle with feeling inadequate, and being able to come in and save the day completely for once gives them back that satisfaction. (Unlikely, I know, but possible)

I don’t understand where this sort of idea comes from… Their rule was a mess, the cartoon never bothered to even try to make a point that Celestia was a half-decent ruler, and the whole idea of them both retiring is dumb, at the least.
 
This is just another excuse to pretend that Celestia and Luna don’t exist so that shit can happen, it’s literally the same thing the cartoon does every season.

I’d enjoy something like this if it involves Twilight being busy with something and leaves Glimmer in charge, which she takes liberty to go full out reckless with her magic.

@ChildOfTheNight  
I hope so

Twilight: Yeah, but just because you decide to retire doesn’t mean that Equestria’s enemies are not going to take advantage of your absence. Besides, I got this!

@ChildOfTheNight  
It’s a good theory

Because nothing bad ever happened under your reign.

Twilight: Oh, okay. Well, here’s an idea for you two. NEVER RETIRE AGAIN!

Thats my theory too i glad more people think this way