Well? Would you? (Expanded below.)
A cynical man might say, "sure, plenty of people pay for World Of Warcraft", but that would be more about people than the game itself. You can solo in most MMOs, but you aren't obliged to. So, cloud-gaming streamed stuff aside, would you pay a sub for a game that didn't have server and admin costs as an expense? (I think there's a racing game and a hunting game that needs subs too, but let's skip that for now.) My question is, more precisely, this: What would a single-player game have to do to make you pay for a subscription?
I got round to thinking about this in one of the hivemind's regular discussions of how content patches and expansions first tried to evolve into episodic gaming, and more recently have regurgitate themselves as DLC. Episodic gaming has been subsumed by DLC to some extent - just look at the success of Borderlands in releasing a main game and then three larger bits of post-endgame content for gamers to play through - but now it's becoming something else: a way of making us pay for content after release. An excellent way of making us want to buy more of the same, and finding a straightforward way of delivering it.
So say this gets standardised and formalised. Say Mass Effect 3 has a piece of paid content coming out each month after release, and you buy each one... suddenly you're paying a sub? An optional one, of course, but it amounts to the same thing. Follow the trajectory of Bioware's DLC experiments and this doesn't seem all that unlikely. You could very easily imagine and endgame for one of their RPGs where you end up doing a "monster of the week" quest now and again, because you were paying $5 a month for the extras.
Extrapolate this: you could conceive of an open-ended game, perhaps an Elite-style game, or a Sims game, in which you trundled about in your sandbox, but pay for the privilege of having new stuff dropped into your game on a weekly or monthly basis. We're used to getting a hell of a lot for free, so such additions would have to be pretty significant, but perhaps they could be sophisticated enough for it to be warranted, and being single player, you could even have a degree of control over it in a way that a multiplayer game doesn't. "I'm ready for my alien invasion scenario now..."
Yes, there's issues: issues with piracy, with the developmental planning and precision required to pull it off, with the risk of doing it at all. But...
Stalker with a content team steady expanding the zone: it's working its way out into the world.
GTA with new suburbs and characters slowly being written in. Genuinely new headlines on the news.
Dawn Of War where new maps and units arrive week after week in an endless war, as long as you keep paying that sub.
Would You Pay A Sub For Single Player?