Detroit had major issues due to it being basically an automotive company town. It was the lack of diversification that killed it.

That's untrue. Detroit had a population of 1.8 million in 1950, and currently has a population of ~665k. The auto industry was still doing well in the 1950s-1960s when the biggest shift out into the suburbs took place. A significant factor was racially motivated, which has since been called "white flight," but what happened was that a huge amount of the population moved, and the businesses moved with them. This passage from Wikipedia hits those points:

By 1960 there were more whites living in the city's suburbs than the city itself. On the other hand, there were very few African-Americans in the suburbs. Real estate agents would not sell to them, and if African-Americans did try to move into suburbs there was "intense hostility and often violence" in reaction.[10]

The auto industry too was decentralizing away from Detroit proper. [...] the Big Three were shifting their production out of central Detroit to escape the auto-union wage requirements. Between 1945 and 1957 the Big Three built 25 new manufacturing plants in the metropolitan area, not one of them in the city itself.