I got two minor things to clear up:Such as they've done for all (relevant) games straight now? Forward-compatibility?
Keep in mind that as a product, XY is considered "sold" already. Most people who are likely to buy XY, have already done so. XY has earned its money. One of GameFreak's most cynical points of strategy is to make each game look more appealing than the previous ones, and that might mean holding back features. If ORAS have features XY have not, that would somewhat diminish the game experience for XY, but again, XY's main sales period ends with the release of ORAS. Once that main sales period is over, GameFreak has no real reason to do maintenance on XY. They will be focused on making people buy ORAS, and in that case, ORAS exclusive content is considered a sales argument for ORAS rather than a detriment to the XY game experience. Once XY has made all the money it's likely to make, it's time to shift the focus onto ORAS making money instead. ORAS is a product they're trying to sell, XY is a product they've already sold, so why should they bother giving XY graphical upgrades to make it on par with ORAS? It's the ORAS game experience they want to make as good as possible (of course, without raising the bar too high for the next game following it).
If they DO patch XY to be compatible with new moves and Mega Stones, there would be a significant amount of players going "meh, everything new to ORAS that I care for will be available to me in XY, so why should I buy ORAS? I'll stick to XY and trade for the new Pokémon". For this reason (and probably others, such as the logistical hassle of issuing a patch to existing game carts without causing bugs), GameFreak have never maintained old games once new ones were shipped. One of the sales arguments for ORAS is "giving you access to stuff you can't get in your already-bought games, so hurry out and buy the new ones too!", just as it was for Platinum, BW2 and to a lesser extent FRLG, HGSS and Emerald. The former three games even put the exclusive content on the game covers just to underline the point, and movies were made showcasing the new Formes (that remained unavailable in the base games).
Yes, it would be technically possible, and probably not even that hard to do. But it would take away some of the exclusivity used to sell ORAS. Instead, GameFreak have tended to ship games with all the content they wish them to be compatible for. Namely, locations, Pokémon species (though not necessarily Formes), items and moves. Planning is done so far in advance that there wouldn't be issues with non-finalized designs such as suggested above. Those deadlines passed way before the release of XY. Again, GameFreak plan their content in advance.
As for balancing Kangaskhan-M, that could be done mostly the same way as Hypnosis' accuracy was debuffed in Platinum, as well as patching the PSS servers so that Kangaskhan was nerfed slightly for XY-vs-ORAS matches as well as online multiplayer. Kangaskhan-M would still dominate in-game in XY, and in XY-vs-XY local multiplayer, but that would hardly be relevant for any sort of metagame. No need to interfere on the cartridges themselves.
First, I should have said "best game experience possible." In previous installments, there was no realistic way they could have prepared for the next games and no way to correct afterwords, so the best possible outcome was few changes between games. Overall, that's exactly what they did, and I commend them for finding the balance between quality and realism.
Second, X and Y are far from done producing revenue. It'll be hard to quantify, but even though they're probably not going to sell all that much, the conferences, events, advertising, plushies, posters, T-Shirts, etc. will all suffer if you make X and Y noticeably worse to play. GF knows that the money they produce by forcing those few sticklers who wouldn't have bought ORAS otherwise to fork over 40 bucks won't cover the loss of revenue from removing XY's value, and that's why they've never made huge preventable changes to new in-gen games. The idea that there are 20 or so megas that you can only get if you pay for a new game strikes me as lazy and short sighted, two things I rarely use to describe GF.