I go by the SF Debris maxim: If it isn’t stated within the show, then its canonisity is dubious.
 
There hasn’t been any explanation in-series for why Celestia, Luna, and Cadence are alicorns, ergo precisely how many there are has yet to be determined.
 
I think the issue here is that fans are imprinting too deeply fanon into canon lore. Don’t get me wrong, there is nothing wrong with speculating on such things, but we shouldn’t let our speculation determine the lore of the show. I personally am going to take a wait-and-see approach to see how (if at all) the people whom make the show explain the alicorns, and expand on the established lore surrounding them

I don’t think Cadence really “breaks” anything, because there is nothing lore-wise to break. The show has not explained anything about alicorns yet.
 
We know Unicorns could raise the run and the moon (and apparently was something of a group effort), so alicorns aren’t necessarily required for that. They haven’t explained alicorn longevity yet. I think it’s a bit silly how some fans are getting upset that their own head-canon is being ruined.
 
It looks like we’ll have to wait and see! :)

@BackgroundPony I could see that. It’s a bit of a stretch, but then so is a show about sentient pony/unicorn/pegasi.

The closest MLP has ever gotten to speaking of Equestrian background and mythos is the Hearth’s Warming Eve episode, and of course the very first episode intro. Other than that, any theories of universe-breaking princesses or whatnot are bunk to me.  
The style of FiM’s writing deliberately leaves its world open to a lot of possibilities. Ideally, I wouldn’t want it stuck in one place because people can’t accept new characters.

What if we’ve been looking at this all wrong? What if Cadance is really part griffon?

@Background Pony:
 
Well,there’s plenty of room for theories.
 
I remember a user in another thread (I think it was at some alicorn OC pic) mentioning the possibility that male alicors don’t have wings because sexual dimorphism.
 
Or maybe that that alicorns ara like the amazons from Wonder Woman,etc.

What?

@Amber and that.

@Amber  
Why isn’t Blueblood an ailecorn?

@TheLoneLampman  
Hasbro still has stuff do clear out. Not everything in the show is going to be a toy, and vice versa. If it’s vaguely pony looking, it will sell.

@Deserter It’s not about Lauren. It’s about how Cadance fits in with the show’s royal family/Demi Goddess system.

I should have also stated that while I didn’t like that they had to do it, I really loved the episode.
 
Now, if you say that the show is crappy advertisement for the toy line (agreed), then why even have the new stuff crammed into it in the first place? I swear, I’m going to blow shit up if I see Applejack driving her truck around.

@Deserter:
 
We are talking about the current showrruners.

Guys hear me, guys, guys, stay with me here, guys, guys, how about:  
Lauren doesn’t work on the show anymore. Shut the hell up.

@TheLoneLampman:
 
You know,looking back,the only thing that was sudden was Shining Armor being Twi’s brother.With Cadance,I sort predicted,because if we had Blueblood being a prince and had of other royal stuff going on,sooner or later was going to appear another Princess.Yes,she’s an alicorn,but at least she’s royalty and a female,which means that they still aren’t suppose to be common.
 
I agree with you with shoving toy line constantly.There shouldn’t be a problem if they do it with subtlety,though.

@HamburgerTime  
Technically, they’ve never even mentioned if Celestia and Luna are immortal or just long lived.
 
But if you listen to the season 3 songs, Cadance is obviously a lesser alicorn than the royal sisters.

@Yorec
 
Sorry one controls the moon.

I don’t think anyone has ever said Cadence is a godess. There might be mortal alicorns in the royal family. At least, that’s what my head-cannon says.

@Background Pony
 
How exactly is it a Yin and Yang system?
 
Why because one controls the night and the other the sun?
 
If anything controlling the sun is a direct reference to the Greek Pantheon.
 
Ever heard of Helios?

@TheLoneLampman  
The thing is, separate people make the toys and make the show. If there were plushies of Angel, Winona, Gummy, Tank, Owlicious, and Opal everywhere, it might make sense.
 
Of course everyone was apprehensive about a third princess, but Studio B once again proved they could pull it off. It could have been done so much worse.
 
As I’ve said before, if the series is supposed to be an advertisement for the toy line, it’s a pretty crappy advertisement.

@TheLoneLampman
 
Oh I admit that the sudden inclusion of those two were jarring but I came to enjoy them.

Equestria wasn’t a pantheon system, though. It was a Yin/Yang system.

For the record, I was pretty unhappy with them shoving in a third princess and suddenly twi’s brother, because they wanted to sell more toys. The friendship express made me a bit grumpy too, as did the pets.  
The Cadence/Shining Armor thing was sudden and does make you raise an eyebrow, but we’ll see where they take it in season 3 and the whole crystal empire thing.
 
Bottom line is, if they have to keep making these progressively larger leaps to accomodate the toy line, fans are going to be unhappy.

Ok fine, explain how she “breaks” the universe then.