Detroit's de-population is a tremendously sad story. The city peaked in the heyday of the U.S. auto industry at 1.8 million people, and is now around 700,000. The decline of the U.S. auto industry meant jobs left the city, and along with those jobs most of the middle class people. This was compounded by white flight in the 1960's and 1970's (and as a result the city proper is 80% black even though the overall Detroit metro area is 70% white). Fully a third of households in the city are below the poverty line.
It's not just the auto industry; Detroit was a US industrial hub, and had a diverse manufacturing employment base that included defense spending, textiles, energy, food... the whole Rust Belt got shellacked by deindustrialization. I might go out on a limb and say that if anything, the auto industry kept Detroit alive. Look what happened to Gary for a more grim example.
It probably stopped the city from collapsing entirely into the dust, which would have been sad.
Detroit was a symbol of what was great about America in the 1950's and 1960's. In 1960, it had the highest per-capita income in the U.S. It was built on American technology and American manufacturing prowess and an industry that brought wealth and good jobs to ordinary Americans, not just the top 1% (either financially or intellectually).
It was built on the auto cartel, which robbed consumers all across the country to keep the Grosse Point estates of the automobile executives well appointed and manicured.
It'd be interesting to see some research on that. Most of the plants I'm aware of aren't actually in Detroit proper, but the metro area.
A quick glance over the list of GM factories[1] shows just one assembly plant in the city. Chrysler has a bit more going on[2] in Detroit. Of course, this doesn't take into account all of the suppliers, which is a huge omission. In my experience growing up in the area, it wasn't so uncommon for someone to be working for a company whose sole customer was one of the Big Three.
It's not just the big three auto plants. There is all of the ancillary plants that make the parts and sub-assemblies. Raw materials, Logistics, Accounting, Foodservice, and everything else that goes along with productive activity.
