Well, the days of AAA sandbox MMO games are probably over.
But there will be smaller titles, and who knows, maby they'll come back into fashion again some day?
But for now, just accept that if you want a sandbox MMO, you'll have to go off the mainstream...
As for Sandbox Games in general?
Well, GTA V is a sandbox game, and that sold incredible amounts of copies, so I don't think its at an end in general... just for the MMO's.
I haven't played GTA V yet, but if it's anything like the previous GTA's... then it will also have tons of story and side missions.
So GTA V doesn't just appeal to sandbox fans, it also appeals to those who enjoy a good story and on top of that it appeals to a lot of racing fans - not to mention everyone who wants to play a bad*ss criminal is also going to pick it up because there are very few games that actually let you do that.
Rockstar has their fingers in many pots.