Gonna need citation. The only thing that got backlash was the cutoff from the previous two games, meaning no one could transfer their pokemon from Gen 2 to Gen 3.Gen 3 was reviled upon launch.
I know there's a meme that "people hated something at release, but like it now", but there's literally no evidence of such. A minority complaining about a new generation is not the same thing as everybody hating it.
Also, saying people just hate a new generation because it's new is such a reductive over-simplification of what a lot of people feel negatively towards a new game.