I think another way to describe the difference between MMOs and TTRPGs is that the former are more akin to mass media, while the latter is more of a refined taste. Think pop music vs. jazz (or perhaps compare commercial smooth jazz to more avant garde stuff). There's always going to be a kind of pyramid, with fewer people enjoying the more refined stuff.
I agree with Neonchameleon (and therefore, by implication, at least sympathise with [MENTION=12037]ThirdWizard[/MENTION]'s points, though I have no real opinion on numbers of players).We need something at least as different as Vampire: the Masquerade was. And something that will cause that type of upheaval in the community. Some of the story games people (Vincent Baker, Paul Czege, and Jason Morningstar in particular) are trying - but it's like trying to bottle lightning.
Crunch heavy games (and yes, every version of D&D fits) are scary.
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Video games have a low barrier to entry. We're having a boardgame renaissance right now - there's no reason TTRPGs shouldn't also be joining in.
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Again this is incredibly insular. A lot of modern RPGs (Fiasco and the entire Apocalypse World family spring to mind) give you tools to invent adventures on the fly. This is another problem with the big bulky book presentation model.
But the problem with such games is that you can sell the game, but then what do you sell next? You can't really sell a product line. You need to sell an entirely new game. Monetisation is a problem.
I'm GMing G2 - a classic AD&D module from the heyday - at the moment (mechanically adapted to 4e, though not in the same way that Chris Perkins adapted it). It's fun, but frankly comparing it to jazz is pretty ludicrous. It's a dice-rolling fest, and all about hacking up giants! (Just as the original tournament intended.)
If I was going to look at games that do warrant comparison to jazz, in terms of elegance, support for improvisation, appealing to refined but slightly edgy taste, I would certainly be looking at Baker, Czege etc. 5e isn't jazz at all, any more than G2 was or is.
Can their games be made wildly popular? I don't know - as Neonchameleon allues to, part of the problem is that mass penetration depends upon publication, which depends upon commerciality, which is a problem for games that support their players' imaginations and improvisation without needing to buy more stuff.