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?