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I said this in another thread and I'll say it again: I've literally never seen a single convincing argument for why megas are bad.
>They're overpowered!/Cause powercreep!
This does apply to some Megas, but only to a small handful, which (apart from fug and maybe salamence and gengar) aren't any worse than box-legendaries. Additionally, there's no reason they can't just nerf the OP megas in the future, like they did with Mega Kangaskhan. Especially if they got rid of the "megas have to increase BST by exactly 100 points" rule. Also, power creep began generations before Megas and continued for generations afterwards. Arceus and Zacian-C both contributed to powercreep more than any of the Megas ever did. Whether or not Gamefreak decides to break their own games is in no way reliant on whether or not Megas continue to exist.
>They're ugly!
Purely subjective opinion. I personally think most of them are at least passable, and a few even look cooler than they're original forms.
>They prevent Pokémon from getting cross-gen evos!
No they don't. There's nothing stopping them from giving new evos to Pokémon with megas, or at least giving alternate evolutions to their pre-evos.
1/2 because comment too long