I think it's mostly nostalgia. Every time I play a modern Pokemon game I marvel at the quality of life improvements. The game just moves along more quickly, you can access your boxes anywhere, you can see Pokemon in the overworld, no random trainer battles, no more forced HMs, etc. I even generally like XP Share, since I'm the type of person that rotates Pokemon in and out of my party frequently, and XP share lets me use a good variety without grinding a ton - and since I rotate a lot I'm not usually super over-leveled.
I would play Gold/Silver again if they dropped, but I feel like I would most likely not finish, if that makes sense. I'm not sure how well those original generations really hold up.
I promise now that I'm not trying to pick on you specifically here, but my opinion can be summarised as pretty much exactly the opposite of this; modern pokemons biggest problem is it's shaved too many of the edges off.
You don't have to prepare at all for heading out in to the wilderness because you have literally everything at your fingertips at all times. You don't have to look forward on your town map, see there's a cave coming up and think "maybe I should bring a few tankier pokemon, there's a dungeon on the next route and I might have to get through a bunch of zubats" or see a long bit of ocean and think "I'll need a water pokemon, and a electric type might be a good idea to deal with the types I'll be attacked by" or when you're heading in to a desert, "A grass type would perhaps be a good thing to bring in to here", because you can always bring everything. My ideal pokemon campaign is one like they used to be designed, by which I mean it should be a list of challenges between major points of interest, to see whether you're adequately equipped in terms of a team in order to get through. And if not, you try again, already a little bit stronger because of your previous attempt. The approach and passing through Mt. Moon, a stage you face after your first gym in pokemon RBY is straight up harder on basically every level, than anything in any of the modern pokemon games, and that's a real problem.
On top of that, there are mechanics that the QOL stuff just outright broke. For example, you no longer have dead weight pokemon that if you struggle a bit through training, it'll pay off later like Magikarp, because you can just leave it in a ball and not require any more work than any other pokemon to train. Like, modern pokemon straight up makes Magikarps gimmick just not work at all.
You have to actively put a lot of effort in to not having a massive strength advantage over the AI, continuously shuffling out these same pokemon who originally, at least part of the idea was you're bonding with your pokemon through your journey.
Maybe these idea's didn't all work out in practice, and I'm certainly not going to claim that pokemon was ever particularly challenging, but they it seems to me like they gave up even the slightest of pretenses at some point.
Certainly in any case, it's hard to buy the pokemon world as dangerous as we were all lead to believe way back when that professor yelled at us to stay out of the tall grass because we'll literally die if we don't have proper partner pokemon with us ourselves.
I guess what I want is an actual journey with my pokemon, that we have to battle and explore through, and I just haven't been getting that from the modern games. Even what should have been an easy slam dunk with the Pokemon BDSP remakes, flubbed it because it had the modern exp curve