Thing is, structural reallignments happen. That's what happened when we switched to a suburban model of city development. That's also what happens as we move from one sort of economy to another (think extraction to manufacturing to service to information-drive).
Detroit is where the city model and the economic switch have led to chunks of the city being quite literally completely abandoned. John Hartigan (Sociologist) has written an article called "The Greenfields of Detroit" discussing the disintegration of the urban core.