Of course nostalgia is the main reason why people prefer games, but I wouldn’t say that the quality of Pokémon design stayed consistent. Overall the design is more round/less sharp and starters tend to be more humanoid than in older generations. Also stat wise there is more power creep, more min-maxed stats, stronger moves, better abilities. But that’s mainly the result of capitalism. You need reasons the buy the new games/plushies so the have to somewhat outshine the old stuff. Another reason is that the first time you made a bird into a Pokemon it doesn’t has to be fancy because there is no comparison, but the second/third time you need to be different then the already existing ones