Now, whether or not saying Mega Evos only occurring in Kalos is to say that they will not carry on Mega Evos to new generations...if it does carry over, by not giving Gen 6 any Mega Evos, the Gen 6 pokémon become isolated. No longer new, no longer capable of /being/ new. Unless suddenly, Gen 7 hauls in and what could not Mega Evo now /can/. Which is being unfair to the Gen 6 pogeys at current, if they can't in their debut but can when they're old news. And also just bullshits on the Mega Evo plot of XY, suddenly having retconned powers. "lol we were just hoolding back."
But if Mega Evos aren't programmed into future generations, that leaves open a gaping hole in competitive play (it shouldn't be going backwards), but in exchange, Gen 6 pogeys rest on the same standard its predecessors have before Mega Evolution (no fancy gimmicks) and the eventual introduction of something New once more to compensate.
All in all, Mega Evos are damned if you do, damned if you don't. A little bit more on that below.
Let's also talk Sylveon. Making its evolution dependent on its affection in Amie, that's cute. But is Amie going to survive into the next games in either this or future gens? Possibly something similar. But why make it dependent on something that might not be in future gens? Look at Milotic: contests lasted two generation, but once it didn't, they had to come up with an alternate solution, so it might be the same case with Sylveon down the road.
Unless, like FRLG and HGSS, what's necessary to evolve some Eeveelutions simply does not exist, forcing the player to either trade for them OR use Bank. Because suuuure, with Bank, you don't need to backtrade; it's all right there, easy-peasy, never have to worry for dex entries again! Good bye magnetic caves, icy and mossy rocks; all them evos should already be in your Bank to dex trade for!~ The same for Amie: you can port Sylveon into Bank, so why have an Amie program just to evolve it? (It's easier to just item-trade, like Milotic, and that will be its fate if the Amie program is scrapped altogether, but there's a slight chance no one will bother with that and just rely on your Bank usage or in-game sighting+GTS.)
Except not everyone wants to use Bank, and not just because it costs money. It's a problematic system (I go into why here
) and one should not have to use it
...but this is going to be the case as future handhelds become "impossible to connect with" a la the DS-3DS tripe. With Bank, there's a true scare of being strangled into using it or else the full capacity of future games could be lost to you, since BW2 and XY show the dexes are getting larger and larger with each region and the in-game box system might finally just quit having just enough room for everything, because you should have Bank: where you can keep things without worrying about in-game box space.
Bank also hurts the possibility of Mega Evos being carried over, because held items are not allowed inside it. Without the option of held items, any future generation game will require all the Mega Stones to be recollected, without or without the addition of more stones to accommodate past gens. I don't know the number of things that Mega Evolve to date (exact or ballpark), but let's say there are 40. More games, more items, always; without condensing Mega Stones into one universal stone, that's just THAT many more items to get. And if
It's a slight mess in any direction of the future you look, and I hope they have a good outline for Gen 7's deal before they put all this crap into motion.