> Cities do not fall from grace like that for no reason
I just told you the most commonly cited reason, and instead of arguing that I'm wrong, you're arguing orthogonal to my point. Detroit became less special as time went on and there was nothing that Americans could do about it - the culprit was neoliberalism. Unions or not, that is the reason why the economy could not persist.
So let me rephrase my question: barring unions or state-subsidized housing, how was the US supposed to prop-up a manufacturing economy in the 1980s?
Government policies were a part of the problem but a lot of Detroit area manufacturing companies were simply not very good at their jobs. They coasted on past success while being unresponsive to customers, and failed to improve on quality or productivity. This was primarily a management failure — only a true moron could approve production of vehicles like the Ford Granada — but the adversarial approach taken by most union leaders certainly didn't help. Union leaders were mostly corrupt and incompetent, acting to win elections and enrich themselves in ways that ultimately hurt their members.
The best thing the US government probably could have done to prop up the manufacturing economy in general would have been to spread knowledge of modern best practices, like those promoted by W. Edwards Deming. Plenty of people were willing to improve but simply hadn't been trained in how to do it. For auto manufacturing specifically, legislators and regulators could have phased in emissions and fuel economy rules more slowly to give manufacturers a few more years to react instead of forcing them to hastily modify old powertrain designs in ways that drove up costs and ruined reliability.