It would be odd to completely remove such a major function from the games entirely; especially since a fair amount of pokemon have these suped up forms now. Of course, with Z-Moves being like an 'Ultimate Technique' learnable by any corresponding pokemon, it feels weird to have both available. I dont think megas will be removed completely, but I do think they'll be taking a backseat to Z moves. We'll probably have to pick between access to one over another. Also, with so many Kanto pokemon getting Alolan forms (possibly even exclusively them if rumors are to believed) then I think megas will go to other generations.
On a side note, I feel like any pokemon that has a mega will not receive an Alolan form; simply because it wouldnt really fit with its prior forms. Like, take Alakazam for example. The rumor is that it becomes fighting type as an Alolan pokemon, but its Mega is surrounded by psychic spoons as it floats in a meditative way and its Special Attack is shot through the roof. If youre raising a fighting version, the setup you have for it is bound to be completely different; not suited for the mega at all. Thats why I really dont think that Mega pokemon wont have Alolan forms and also why I doubt the existence of an Alolan Abra line. Unless they just make it so that pokemon cannot mega evolve or something.