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 dunno man, Star Citizen has already reached 34 million in funding. Keep in mind that they don't have to go through a third party like EA so most of that money will go to funding the game directly.
Then there's always EVE Online... quality-wise it's definitely "AAA."