Having nostalgia to something mid by itself isn’t a bad thing, and more power to those who genuinely enjoys the game, comic, movie or what else they grew up with. It’s only when they started to defend both the good and bad thing about the stuff they grow up with that it becomes a severe issue as it comes off as insecure and disingenuous, and it can turn into a vicious cycle within a fandom with a vicious debate of which generation is the best one, to the point some fans called the current the worst one yet only to swiftly change their mind, and vice versa.
I have yet to see a fandom that were a bigger culprit than Pokémon, even beyond the game side of the community. I do not think not even the Sonic fandom, Dragon Ball fandom, FNAF fandom or other noteworthy fandoms were even close about it. While Kanto is pretty much “patient zero” regarding several Kanto fans, it is safe to say that many Gen 2-7 fans are also victim to their own fallacies even before the whole Dexit situation, and something tells me it might also happen to many Gen 8 + 9 fans as well.
I have personally found the opposite (not that Pokemon isn’t bad in this regard, but there are many others just as or worse)