It's kind of sad most of Ash's series was the same for most of its run and now Horizons is the most plot driven the anime has ever been with twists and turns and well done villains and developed protagonists. It's the kind of show I would have wanted decades ago when I was a teen growing up watching the show. Granted Horizons isn't perfect, but it's a step in the direction it should of gone.
Again, I can't really compare it to Horizons, although I do think it sounds like you're hyping it up a bit too much here, but I wouldn't say that the anime was mostly the same beforehand. It wasn't like you've seen one Pokemon series you've seen them all prior to Horizons or even SM. Each series still felt distinct, still had different characters, group dynamics and storylines to make them stand out. They were formulaic sure, but that isn't the same thing as being the same. You wouldn't get the same experience watching an episode of AG with BW for example. Maybe that still wasn't different enough since Ash was still losing Leagues and starting over with a fresh team each series, but I still think it was different enough for what the anime is and it was still enjoyable.
Thinking about, it most of Ash's series is mediocre or hasn't aged well. Kanto/Orange are great but of course the battles were the weakest of the series due to the writers not really being sure how to write battles yet. Johto needed to be 10 times shorter, as the good eps it does have is overshadowed by all the boring episodes. AG is now just fairly average, inbetween the extremes of all the other sagas. DP was mostly good, but hit by pacing problems. BW had a forced reboot and some really weird/questionable writing decisions. XY was solid, but re-treaded a lot of the previous series and the fillers were so-so. SM was fine for what it was, but would have worked better without Ash. And JN was just a mixed bag.
I heavily disagree. I still enjoy each series. Even BW was more enjoyable when I rewatched it a few years ago despite being my least favorite series. The Pokemon anime is a big source of comfort for me, so just getting to see Ash, Pikachu and his friends going on adventures to see new regions and Pokemon is still fun. That isn't to say that they're absolutely flawless. I have major issues with just about every series, but to say that they have been mostly mediocre or haven't aged well is doing a pretty big disservice to them. It certainly feels a bit too harsh to me at least.
Considering how much you've put May on a pedestal, I'm kind of surprised that you're calling AG average. Admittedly, I can understand that more when other series have bigger highs than it, but the series itself was still enjoyable and had plenty of good moments in spite of its problems. I still think dismissing XY as retreading stuff from previous series isn't quite fair, or at least I don't think that would be an issue for the actual target demographic. I still don't see how SM would have worked better without Ash. It would be a different series without Ash and Team Rocket.
Thinking about it, Ash's series has 1,200+ eps overall, and only like half of those episodes are even good. Granted imagine if they did a "Dragon Ball Kai" cut of Pokemon, only keeping the best fillers of each series and the most important eps, and that would ease some of the problems. Granted Ash losing leagues for 20 years wouldn't be helped.
If you really think that only half of those episodes were good, I genuinely don't understand why you've watched the anime as long as you have. Even when most of these series were airing, you kept saying that this was going to be Ash's last series pretty much until it happened. I'm not saying that you or anyone else has to like every episode or anything like that, but if so little of the series actually is good in your opinion, then I think that's a big sign that the anime just isn't for you anymore and hasn't been for a long time. It's certainly a huge sign that you're about three decades outside of the target demographic too.
A Dragon Ball Kai cut of Pokemon keeping the best filler episodes would be contradictory since the point of DB Kai was to trim down the filler from the original series to make it more manga accurate. It also just wouldn't make sense to do something like that for Pokemon when they're rather focus on promoting the newest games instead of trimming down older series. Besides, fans could easily do that themselves by just choosing which episodes to watch instead. That's what episode guides made by fans typically do. Granted, I've never been a fan of those since the Internet has really misused the term filler to the point where they don't see anything not directly to the main plot as important or noteworthy, but my point is that people can just skip around older series to see what episodes they'd want instead.