Adjacent to that, I'm a bit surprised that there hasn't been a mass rollout of assisted GNSS in large cities. Maybe not full RTK, but something more than just delivering almanac being delivered.

Are the current GNSS solutions good enough (or too degraded?) in city jungles that there would not be any mass benefit? Too expensive to run a few base stations here and there?

I think so, yes. That's the main purpose of RTK

Yes. WAAS should also help a lot, or at least tell you that you can't get a good position.