But who would want such a device? I can get a heavily-subsidized Google Play-based Android phone from my cellular provider for practically nothing. An AOSP fork will be missing all the Google Play goodness, and I highly doubt it'd get subsidized to any extent, so as a consumer I'd have to pay more in order to get less.

Yup. I figure that the current crop of manufacturers are going to have to be pared down a bit, as margins are just too low to support everybody. Apple's bubble is eventually going to pop as well; they've managed to milk their mobile device boom quite well, but nothing lasts forever. The iPod has certainly peaked long ago, and I doubt the aWatch or the aTV are going to be anything like the iPhone in popularity. And a shrinking market is going to make it even harder for new players to enter...

In short, I don't see why the common consumer would want (or care about) a new mobile OS. Not unless it did something that the existing OSs don't do, or don't do well. And there's very, very little that the existing OSs don't do...