Mearls and/or Crawford have stated multiple times that Core+1 is their design philosophy for 5E in general, not just as an AL rule. They have said they want people to feel that they do not HAVE to buy more than the PHB to keep up with other players and to have fun They do not want it to be like 3.X or PRPG where it feels like you have to buy 20 different books just to build a character that feels equal to other PCs.
And as for AL play, just to remind those who have forgotten, or never played AL, the Core+1 was for each season of AL play, where you chose an origin for your character. So it was Core+Tyranny of Dragons or Core+Princes of the Apocalypse, etc. I think they changed the +1 equaling an origin to the +1 being for a specific supplement after Volo's was released and they realized origin was now meaningless.
And even when they finally release a book that contains new, full character classes, such as the Mystic and Artificer, I doubt they will change their philosophy. But the AL may have to change their rule again to account for those new classes. And for all we know, any new, full classes may just be declared a part of Core, regardless of which future book they are in.