Then how do you account for Mega Sceptile and the others not being in XY? It looks like they did half-ass both solutions at the same time.
(...)
We should all think that if someone brought up something such as mega Diance exactly one week ago the first (and plausible) objection would be: lol there was no diancienite in XY data, they plan this way in advance and no 6 gen pokemon got megas! Yet we all know what happened a couple of days later.
There would be no Mega Sceptile or Diancienite in XY for the same reason there was no Griseous Orb in D/P, or no Reveal Glass in B/W. They aren't essential for the new Pokémon formes to work in battle against the base games. As long as the Pokémon has a base form recognised by the base game, it can be displayed on the base game through trade or battle. A new forme is simply a new combination of old elements (species, moves, abilities), plus a new sprite/model. And as long as there is an old sprite/model to display, the game has no problems with the new forme. Problems only arise when entirely new elements are brought into the picture.
Besides, one of the main selling points of new Pokémon games has always been the allowed access to "new Pokémon" (at least since gen. III). Want a Giratina-O? Well, then you have to buy Platinum. Fancy that Kyurem-B? Buy BW2 if you want to use it. Your friend may very well use the Pokémon against you (because his game is compatible with yours), but you can't have it yourself if you only own the base game. Ask your parents for the new game today!
It would be literally exactly the same with new Mega Pokémon. If you want to play with the Mega, you have to buy the new game. No mystery here.
In short, GameFreak wants the new games to be
compatible with the base games, but they do not want all their content to be
available for use in them. New moves or abilities screw over compatibility as well as availability, which is undesirable. New Pokémon formes (including Mega Evolutions) would be compatible, but not available.