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 agree, most PC MMOs today seem to follow the WoW route with a theme park levelling process, although SWTOR is very much theme park heavy compared to WoW. And I think this game lost so many subs last year not only because of the levelling process and maybe because of the story because those people may not have been expecting so many cutscenes or have never played kotor or a bioware game before...I reckon most of those people started playing this game because it's got the star wars logo on it.
But anyway i don't know many mainstream sandbox mmos today apart from EVE online and runescape, but they seem to be newer ones coming out such as TESO and the stomping land and dayz