Megas at the very least feel like a natural progression. A further evolution to even Pokémon in their third stage is something that everyone had imagined at some point, and treating them as temporary form changes avoids damaging the pre-existing limitation of the hard 3 stage limit. In that way, Megas are more like super forms, but tied to the existing term "Evolution" which makes it far easier to swallow. If they just called it "Super Form" it wouldn't be as accepted because super forms aren't a thing Pokémon really showed before. They showed alternate forms, and super forms in other media are popular, but I think it would be under more scrutiny for Pokémon.
Z-Moves are super moves and Dynamax is "they get big", and they all start to feel like gimmick nuke buttons rather than permanent evolutions to the franchise.
