As much as I hate to say this, the fandom has always been problematic in every gen since, I believe, gen 3. I'll admit that I didn't see any negativity in gen 3 as I was too ignorant of social medias that time, but I've heard stories about it. Gen 4 was ruthlessly brutal, I've seen people hate both Chimchar and Piplup so much along with just about any of the new pokemon revealed before the game was out. And I'm sad to say that I was one of them, I was on the gen 4 hate train (until I finally cracked and decided to get the game a day before release lol).
I think the actual problem is that social medias are getting louder, and not because of gen 8. It's become much easier for people to spread anger and misinformation, it's a shame really. The best we can do is ignore them and try to spread positivity.
In the end, the best thing to do is stay away from the negative group and only see what you enjoy about the game. Because, the reality is, they are actually a very small...but vocal group, and a lot of them are trolls looking for fame as well. Don't let them ruin your game experience, just enjoy the game.
Now back on topic, I can't imagine the 25th anniversary having anything significant for pokemon as it can consume a lot of resources every 5 years. I would imagine that they'd want to save their efforts for later year anniversary. I can still see a new mainline game happening before gen 9 though, but it's definitely going to be interesting as there are several different possibilities.