That's when Mega Evolution was introduced, which started a trend of introducing a shitty battle gimmick every generation
It also started to casualise the series. Megas (along with the new Exp Share, easier difficulty, Z-Moves, ect) made the singleplayer easier than it already was, with the worst case being ORAS with it's shitty AI

Pokemon Black 2 & White 2 were the last good games in the series...