What is unsustainable is our industrial/trade policy. You know, the one that turned the most powerful engine of manufacturing in history into a bombed out rust belt. Where the rust belt is shrinking, such places as Shanghai, Shenzhen, and Beijing are rapidly growing. That's no coincidence.
No, cities are not self-destroying. How old is Rome? What Obama is trying to do (it's actually not his idea but he likes it) is a classic communist/fascist doctrine of controlling populations and people. Stalin, Hitler, Mao, and our own government with the Indians have used it to powerful effect. Did any good ever come of it? Forcible resettlement. That's what it's called, and whatever phony good intentions are used to justify it, it's still bad.
Also, it won't stop with a few places like Flint or Detroit. Bank on it.
This is an important point, but I would say the politicians and unions in the midwest share much of the blame due to their complacency. They were only concerned with the status quo, while other regions of the United States became the new centers of investment and development.