while i think most people like the 'idea' of a sandbox, i don't know how many of them are truely willing to actually use them. I played SWG from about a month after launch until the NGE finally wrecked it for me. yes the planets were sandbox and you could go anywhere vs SWTOR where you're on rails more or less each planet. the thing is - nobody really ventured way out into the nooks and crannies of the SWG planets much at all.
as someone who always was out there in the hinterlands placing harvesters and running my economic empire, i rarely ran into anyone else out more than a couple thousand meters away from the spaceports, except maybe other harvester planters.
my point here is.. just because the idea of a sandbox is appealing, even when one is in front of gamers nobody actually takes advantage of it when they have one
I played SWG from Beta to death and I have to say, people went EVERYWHERE all the time. People were always looking for that awesome view for their houses, looking for random POI's, or the perfect place to set up a Town. Crafters were always venturing deep into the Wilderness for the best resource spawns...
I even know that on my servers (Scylla and Starsider) people had Guilds dedicated to exploration of the maps. So, when you say that people didn't venture off into the unknowns... That's not exactly true at all.
As for the Sandbox Game theme going downhill, that isn't true. Look at Everquest Next, The Repopulation, Wild Star, and Star Citizen. All games coming out.
Theres also Age of Wushu, EvE, and ESO on the way. (ESO is less Sand boxy, but still more so than TOR is.) So I would not say that Sandbox MMO's are not around anymore.