I don't think you're giving the parent enough credit. People didn't leave North St. Louis because of economic decline. There was never any such thing. People left North St. Louis for the suburbs on their own accord -- for lots of reasons. They wanted the new housing that the suburbs had to offer. They wanted to be further from blacks. The car made for an easier commute.
This created an oversupply of housing in the region. St. Louis doesn't have too many houses because people went to other cities. It has too many houses because people abandoned the housing in the urban core and built new suburbs. That obviously did cause a loss of economic opportunity for the people left behind. But it didn't happen the other way around.