On the contrary, I think it's nostalgia what is ruining Pokémon. Or more like the need some fans have for it to be appealed to them, and I think Gen 5 and Gen 6 are a good example for it.
BW and B2W2 are best known as the best main series Pokémon games ever for many people and for some good reasons there are. They are also known for both being comercial failures (B2W2 in part because ppl don't know the difference between a sequel and a third version). That... doesn't add up at all, right? BW was a totally new experience to put it that way, all of the Pokémon here were totally new, not even Pikachu on sight as weird as it may sound. It was the first time GF focused more in the story than usual. Of course, Pokeboomers wanted Mr. Lizard in their teams, but instead got "le ugly ice cream and trash pokemon lmao" and began saying shit like "Pokémon stopped being good after Gen 3" and whatever stuff I believe everyone has read at least once, which obviously ended with the games having poor sales.
After that came XY, which are often criticized as the worst Pokémon games along with USUM and SwSh, but they somehow sold better than Gen 5 games and the reason is simple: When XY was released, people didn't see how bad the story was, they saw the fact that you could choose a Kanto starter and that Lucario (or their favorite generic Pokémon) could mega evolve. That concluded with XY being the second most sold 3DS game until 2018 even if Kalos is only remembered now because of the anime.
So no, nostalgia is not what makes Pokémon good. It is only part of what makes Pokémon as successful as it is, and TPC knows how to exploit that well.
What makes Pokémon good is that even if they are afraid of going away from the main Pokémon formula, they are still able to innovate in some way. I'm talking about things like the stadiums, the trainer cards and the Wild Area in SwSh (not Dynamax, fuck that shit. I don't like how these gimmicks were introduced since XY). That shows that they can still do their thing correctly and that the only reason the games aren't as good as they could have been is because TPC likes Christmas too much to think about delaying their games.
P.S. Fuck Charizard and Greedent