It would seem extremely odd if they introduced a major mechanic to 20-30 pokemon only to let it die in a generation. Which is my concern with the alola forms; only one generation will feature alola, so does that mean the concept of "regional variant" dies with this gen, or will there be "not-Alola" forms for the future regions?
Presumably the regional variant concept will continue in future games, but feature variants specific to those other regions.
GF always makes things, sometimes great, sometimes not, that they just discard for the next generations, but those are usually minor things; to discard such game-affecting mechanics, with all the work they mean in design and conceptualizing, would be really strange, even for them. (not to mention, in future games, how would you get an alola form if you're not in alola?).
Two possibilities there:
One, Alolan-forms are not necessarily exclusive to Alola. Just as those Pokemon were first brought into Alola, it's entirely possible that the resulting Alolan-forms could be taken out of Alola and then flourish in other regions. Alternatively, it is conceivable that the Alolan-forms, while refered to as Alolan-forms, are not necessarily exclusive to the Alola region. In theory, similar changes could have appeared in the same species if they were exposed to similar environments.
Two, good old fashioned trading. Not in Alola? Want an Alolan-form Pokemon? Then trade from previous generations!