this is maybe a hot take but I feel like the horizons fandom is arguably worse than the Ash series fandom ever was
like I'm not by any means saying the Ash era Pokémon fandom was all sunshine and rainbows, I witnessed a lot of arguments, dramas and whatnot in my time in it and even got personally harassed by a shipper for not liking their favourite ship, the fandom was horrible in certain places
but the horizons fandom seems to have all the same issues the Ash series fandom had and more
everything that the Ash series fandom did, the horizons fandom do too, and anything the Ash fandom didn't do, the horizons fandom somehow do twice as much, just with different characters or ships
pokeshippers and amourshippers who harassed people in the Ash fandom have been replaced with friori and likodot shippers in the horizons fandom
while the Ash fandom was known for hating on female characters without any reason, the horizons fandom is just as bad for that, while also hating on male characters twice as much as the Ash series fandom ever did
and those are just two examples of many
I also think the horizons fandom has far less positive spaces than the Ash fandom did. I have great memories of positive and fun interactions and conversations from my time in Pokémon journeys fandom spaces, while all my interactions and conversations in horizons fandom spaces have been nothing but negative and draining to the point it caused me to stop watching the show and to leave the fandom
honestly, as much as I think that it was time for Ash to stop being the protagonist, I also wish he'd stayed protagonist forever so that those positive spaces I knew from before would still be active now and so interacting in the Pokémon anime fandom wouldn't have become as awful as it has today
things weren't amazing back then but they were better and I really miss that
I'll be blunt and say I agree with you - it's part of the reason I'm no longer really active in the Pokehori fandom. The other part of it, I will admit, has to do with the writing direction of the series, but that's another conversation.
If I had to speculate as to the reason for this change, I'd say it's a combination of the rapid increase in mainstreaming online fandom spaces, lack of tagging etiquette, a dip in writing quality, and just time. The XYZ/Kalos era was around the time that Pokémon fan spaces became really mainstream, and we started to see the hallmarks of shitty fandom behavior, but I never actually encountered any of that personally; I only know about it in hindsight, from others' experiences. In my case, the "fandom" I was part of was a collective of maybe 30 small Pokemon AMV creators, who were kind, friendly, and respectful. It really felt like a community. Between then and now, Anipoke reaction videos, theories, clickbait, and shipping became a lot more vocalized, and people were able to communicate more directly with the creators through Twitter. And between then and now, a whole generation (or two) were introduced to fandom spaces with little to none of the past built-up etiquette, which in turn lead to the resurgence of ship wars and just overall unconstructive negativity.
I do think this would've happened even if Ash had stayed the protagonist, though. It's just a clearer divide because we have a whole new cast, but I recognize much of the negative behavior in the Horizons fandom in its predecessor, the Journeys fandom. It's just louder, and more present now.