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Well with these new mega evolutions I'm thinking they used these mega evolutions to solve the problem in gen4 where we had so many past gen evolutions. They won't take a spot as a pokemon in the pokedex, so they can focus on making the 100+ mons to be new. It can also act as a way to competitively balance pokemon. With mega evolution you keep the same Pokemon and give them better stats which some pokemon need like Mawile/Ampharos.
What are your thoughts on balancing old pokemon through form change?