I remember when UO came out...I never played it but I wondered if it would be the end of good single-player RPGs. Luckily, it wasn't, but it was basically the end of Ultima as a series, though arguably it wasn't a causation. In any case, it's hard to imagine an RPG world that could be designed successfully to be a MMO and a good single player experience.
Good single player experiences are bespoke storytelling affairs...and to abstract it out to accomodate MMO interaction would seem to necessarily dilute it.
Maybe I'm in the minority here, but I'm not at all against games that just end, rather than try to shoot to be a world in which you can fully "live a life" and in which the world is infinitely evolving. It's not that such a prospect isn't interesting, it's just that it inevitably leads to the kind of feature creep that leads to bugs and outright brokenness (insert reference to SimCity). I'm OK with games ending after 50-100 hours (or even 10, in the case of the Portal series), if those hours were fantastic.
I'd contribute if Lord British were going to make a modernized version of Ultima VII. Even though the limits of its world are obvious in retrospect, it really was way ahead of its time in creating a believable digital world for the single-player, and the constructs it used to maintain that facade (and have a story arc) would not have worked if it were a MMO.