As far as decay, this has been a long term thing. Outside of major urban centers where people rely on public transit, most of America is based around car culture. To sum it up, Main Street USA didn't die because of big box retailers, it died because the parking sucked and the affluent went where their cars took them. If the buildings are ratty, we build newer, cheaper ones and drive to them.

This is nothing recent, it started in the 50s and 60s with white flight and has been on a slow, slide downhill ever since because of cheaper constructions methods and cheap gas. Take those two things away and Americans just might go back to monolith building, if for no other reason that the need to work.