Earlier quoted context omitted.
There's an irony in that there is often a regulatory issue with the local government, not just some "shortage of humans". The same thing happened in one of the most populated regions in the US. The area between Boston/NYC and DC, containing Baltimore, was once a city of a million people. It contains an underground subway and was once referred to as the Paris of America, however through regulation and unions it became…
The history of Baltimore is a far more complex story than "regulation and unions" results in "and the city collapsed."
The rust belt as a whole was primarily just companies escaping unions. Cheaper labor may have been an incentive but it fundamentally started with companies who were forced to move production, and US regulation which prevented them from benefiting from local supply chains once they became unionized.