Now that I think of it, and in the risk of sounding like a hypocrite for not liking unpopular opinion threads, I want to say that I get why Megas, Z-Moves and the overall amount of pokémon are getting removed from this game.
The competitive scene was already messy with monsters like Landorus-Therian or Heatran around, and with stat boosts and deadly moves on top of it, it really just boils down to a broken competition.
Of course, that doesn't mean that limiting pokémon is a good thing, since these games should always be "catch first, fight second" and not the other way around, but from a competitive standpoint, I can get why it may be necessary to do something like that.