>It would probably take decades to rebuild a car-centric city for affordable housing and walk-ability. It would be easier to start from scratch, for sure.

China is able to build entire, gigantic, "ghost cities" within a few years. And bulldozers and explosives don't take much time to clear land of existing structures. If there were actual political will to change existing cities to be walkable, it could be done in a very short amount of time.