you're forgetting the upcoming alleged Pokemon Z (it's not just X and Y that are Kalos) but on top o that, how many pokemon have been "fully" evolved in a game just to have the next gen come out with another evolution? (Magmar used to be it, then came Magby, then a couple gens later came Magmortar, it will be hard to give EVERY pokemon line a mega evolution unless you stop it from ever being able to evolve a normal, permanent way. (so if they gave Farfetched a mega evolution, or Kecleon, or Alamamola or whatever, those pokemon would never get an evolution beyond what they are. It took 4 generations to give Magneton it's evolution to Magnezone, or Lickitung's evolution to Lickilicky, or Rhydon's evolution to Rhyperior, so how many pokemon is gamefreak willing to cap their permanent evolutionary lines forever just for the sake of a temporary mega evolution?)
I'll be honest, I am kinda hoping that mega evolutions are a one off thing and it's gotten rid of in Gen 7 like the weather effects, or seasons that they seem to have abandoned as game mechanics. Yes, it is very cool, it's very fun, but it's too Digimon for my liking, and I would much prefer a permanent evolution that I can train for rather than temporary evolutions that you not only don't know the stats of unless you look them up online, but in some cases completely changes their stats (such as Mewtwo X becoming a physical attacker, so all teh training that you normally do for a special attacker is wasted because now it's suddenly a physical attacker in battle) granted, if it just made their regular stats better I might have a different opinion of it, but since some pokemon are fundamentally changed from their normal evolutionary forms (such as the previously mentioned special attacker becoming a physical attacker), I like the game mechanic much, much less.