The information age killed the industrial age for one thing. The other reason was that the domestic automakers globalized because of he racism and greed among the working population of Detroit and most of America is what I saw.
After the race riots in 1967 Motown started to die. White people, and the property base moved to the suburban counties. The core city got racially and economically segregated. The whole city became a ghetto. Detroit was never rebuilt -- they just moved the ghetto to a new location > most of the City of Detroit. The upwardly mobile Black people moved to the inner ring of suburbs so the Black middle classes deserted the city also. This left the mainly the down-and-out population to survive.
The American worker was becoming a pain-in-the-ass. Why pay a UAW autoworker $60/hr with benefits when a Mexican auto worker will kiss your ring for $20 a day?
There are still domestic assembly assembly plants in the USA but the auto parts industry -- that is where the Detroit area workers (office and clerical too -- what's a typing pool today?) and businesses made a lot of the automotive industrial revenue, is a shadow of what it once was. There once were hundreds of smaller non-union and unionized job shops building tooling for auto assembly lines. More work that was done by semi-skilled labor is done better by automation machinery now also. The machine tooling industry needed less skilled workers like millwrights and machine builders. The computer age came and there were new generation jobs for electronics techs and engineers.
The ''conservatives'' wanted to ''conserve'' their old industrial jobs and refused to learn new post industrial vocations. So, the jobs moved and the smart people did too or learned to work apart from the auto industry. If they didn't; they get food stamps and flip burgers at McDonalds or work at Walmarts. They conserved themselves into poverty -- they and their children are the new underclass.
If auto production was 100% USA parts and labor produced from machinery made in the USA a Chevy truck might cost $75,000 new. A new Cadillac might cost $200K. The Automakers would price themselves out of the market. The McJobs bunch would never be able to buy a used car -- sort of like a down and out Somalian I guess.
Bottom line -- Detroit fucked itself with its own greed
I lived in the City of Detroit 1955 to 1974 I watched it dying then GFO.