Detroit destroyed itself by building crap cars.
Period.
I would just add that it was by sucking up to unions and protective politicians, rather than customers--and therefore building crap cars.
Tariffs on foreign manufacturers like the Japs made Detroit grow soft, arrogant, and stupid as well. People don't realize that when you throw around welfare, whether it's to individuals or corporations, you damage the hard-working people and put them at a disadvantage. Such as the American innovators in the auto industry who weren't needed or funded in the fat-union days of Detroit.
Then the tariffs went lower and the Japanese cars, which were better engineered and more reliable, came in and ate Detroit's lunch. The American auto industry, which created and built the Hellcat fighter--which made mincemeat out of the formerly untouchable Mitsubishi Zero--did not need to be destroyed this way.
It will return if we defund its obstacles and parasites.