As I already said, we're missing 35 total megas between the ones in XY and ORAS.
We've had the same EX cards since BW. Unless they replace this gimmick, which seems unlikely, even when we move onto the next generation it is very likely they will continue to make new EX's and possibly megas. The megas themselves are probably something that will remain in the franchise, both game and cards, for a long time to come.
Ever wonder if they should have handled this differently? I mean, we already had a Kyogre EX and Groudon EX in Dark Explorers and Rayquaza EX in Dragons Exalted, not to mention Latias EX and Latios EX in Plasma Freeze. Rotation reasoning aside, was it really necessary to remake them when instead they could have just gone ahead and made the Primals or Megas instead as some kind of collection a la M. Diancie EX collection and saved that room for megas who have no previous EX form? Really, how easier would these megas have been to get out if it weren't for the rotation, and the inability to use past EX's in the R/S/E sets. For that matter, aside from the fact one can't really make evolution decks these days and expect to win apparently, why megas even had to be done in EX form as opposed to just another stage of evolution in the card game? I mean in the video game you don't just start with a Charizard, you have to raise it from a Charmander and then get the mega stone on top of it. I know it's not entirey fitting for a card game, but to evolve from basic EX Pokemon seems just as extreme.