Two thoughts:

1. People will move back to cities as cities become more attractive (thanks in part to things like the shared economy). There's already some evidence of this trend among millennials. Reverse white-flight if you will.

2. Uber will eventually work just as well in suburban areas. The fundamental economics don't change (higher resource utilization => lower costs). It will take longer to get to penetration levels that result in sufficiently high utilization. But we'll eventually get there.