You know who claims the fandom is dying? The young ‘uns who are in their very first fandom (or at least in their very first non-video game fandom) and who don’t know anything about the pop culture of the 20th century and its fandoms. They can’t imagine that a fandom of a series can survive after the series itself has ended. They simply don’t know that many fandoms that are even bigger than the Bronydom have survived one or even multiple series ends with no new material whatsoever coming immediately after them.
Best example: Star Trek. By these Bronies’ logic, the Star Trek fandom would have ended in 1967 after the first season of TOS. But the early Trekkers had it un-cancelled twice over before the plug was pulled for good in 1969. Did the fandom die then? No, it didn’t.
Second-best example: My Little Pony: Friendship is Magic itself. When the show ended in spring 2011, there was no word about a continuation. It was over, full stop. Season 2 wasn’t announced before summer. But the fandom didn’t die. It didn’t even slow down. Instead, it grew like crazy. (Not to mention that we survived the first half of 2013 with Twilicorn and Equestria Girls.)
What certainly is true is that the fandom is smaller than it was around 2013/14. But that’s because the hype is over. The Bronies we’ve lost were mostly those who jumped upon whichever hype they deemed the biggest, just to leave us for FNAF/Steven Universe/Undertale/Rick & Morty/what-have-you when it became the next hottest thing.
The idea that thousands upon thousands of long-time, dedicated fans will take their leaves just because the show is over is complete and utter non-sense.
Also, this fandom is too big to simply die out completely this quickly. It will survive a mass exodus of the aforementioned unexperienced youngsters under the influence of a self-fulfilling prophecy. It will survive G5 being complete garbage (because many will stick to G4). And I think it will even survive the next great shows that will come for us and lure fans away from the Bronydom.
I guess many don’t even know how utterly gigantic this fandom’s footprint is. It cannot simply vanish. In order for the Bronydom to completely disappear, it isn’t sufficient for a few thousand fans to quit. Millions of fans would have to leave, Web sites like Derpibooru, Equestria Daily, FiMfiction, Pony.fm, Ponyville Live! etc. would have to be shut down and deleted entirely, all dedicated Brony conventions would have to end, the Ponycons would have to become collectors’ conventions again like they were before 2011, and every last former Brony would have to delete all their works from non-pony sites like YouTube, Soundcloud, Bandcamp, deviantART, Furaffinity, Pixiv, Tumblr and FanFiction.net. And so forth.