I guess, so maybe Volcanion could get a new forme in a potential sequel.The way you're wording this implies there was a point in time when this was not confirmed. It's been a given since day one that actual new, Dex-expanding Pokémon come only with the new Generations.
That aside, it's not a given that Megas are all we'll get. The existence of Hoopa's new form confirms that the concept of forms isn't being wholly replaced with Megas, so there's a possibility of seeing them, too.
Also, I cant find the interview, but I think one of the producers said something about not really having a plan for an XY sequel, which would make XY the shortest generation to date.