Ok, well I suppose it depends on our own definition of successful. I'll just amend it and say a game that primarily focused on forced grouped content will be not be a widely popular game.
There are other factors in Warcraft's success besides less group demanding content. Mostly it was a matter of ease [everything in Warcraft is simple to understand, discern, very clear, very focused] and accessibility in the form of speed of play [Warcraft is fast] and ability to run on a Ti-56.
Culture also plays a part, natch.
It's very difficult to say that anything will reach that point again. Was sort of a zeitgeist thing with Warcraft.
Though that isn't to say that new games don't have large group content; Wildstar is going back to 40 mans, for example. Or would not be successful from a financial point of view. [Yes. I know you are not intimating this directly, Doozerjun.]
Bovinity is right here; the perception of "successful" is pretty skewed in the shadow of Blizzard's monolithic MMO.