You are clearly better placed than I to know.What killed Detroit was the civil war that broke out.
Heh... Yeah, and it's a tough distinction to figure out for outsiders. Detroit and Metro Detroit have been two very separate things for a very long time. Yet, Metro Detroit is often referred to as Detroit, as well.
Detroit can mean the city or the entire region from Monroe (South) to somewhere out by St. Clair (North) to Ann Arbor (West) to Canada (East). Us locals know the difference between Detroit and Detroit. And, if you were driving through, you'd know when you crossed that border, too. It's a pretty marked difference, and it's still getting worse in lots of areas.
In the early 60's, as urban sprawl was kicking off all across the US, Detroit was no different. Large manufacturing businesses were moving outward for more room and less regulation. Then came the riots. Detroit has always been a bit tense on race relations. Dating back to the Underground Railroad, we have a large African-American community. And the riots of '67 exacerbated the situation. The white people left the city for the suburbs. In droves. (We're talking from 1.6 million in 1960 to 200,000 in 1980.)
At that point, you've got some big problems... It stands to reason that not as many people moved into Detroit as moved out to the suburbs. And the people who did move in earned less, as the people who moved out were already solidly established. Politicians and Mayors furthered their own agendas by driving a wedge between Detroit and the Suburbs, riding the thinly masked race card. The whites in the suburbs were largely of the opinion that the blacks were useless freeloaders who do little but drive down property values. From their point of view, this was very true, as they just ate huge losses on their homes moving out. The blacks were of the opinion that the whites in the suburbs were trying to own, repress, and exploit them. Which, from their point of view was also very true. The riots were spurred by a bungled police raid on a group of blacks led by a largely white police force, led by a white mayor... Thus setting the stage for Coleman Young's runaway popularity. He wasn't afraid of whitey.
Moving forward, the whites basically kind of ignored the city of Detroit, and went back about building new communities around the rim of the old city... (They could do this, they were already established...) While the residents of the city didn't want any part of the "outsiders interfering." Sadly, this largely meant they cut off their own economy and chances to prosper. And that's how and why the borders got drawn up. And they've stayed there. During the boom times, you could gloss over things like that... Wayne County and the State of Michigan could float the city. But it's clear that is a sub-par arrangement. And in bad times, it's gonna crumble. And here we are.