I'm skeptical that the impact would be that much. It certainly would be terrible in Michigan, considering the direct jobs of course, but also all the service industries. But many of the other "indirect" businesses would seem to be able to either a) convert operations to foreign cars, or b) rely on warehouses of parts, and/or the retention of parts-manufacturers to survive long enough to transition. If there are 1/10 jobs, or 15 million related to the auto industry, it doesn't mean 1/10 are related to just the big 3.
The hardest hit direct business would be dealerships. They couldn't convert to foreign as other dealers already have that market. They'd have to find new jobs. The other largest direct is rental agencies, but they could presumably convert entirely to foreign cars; labor/parts on existing cars would be the problem. Small businesses using big 3 trucks and equipment would face the labor/parts problems too, along with truckers, public works autos, etc.
But, the parts/labor thing could be solved (I guess) by relaxing or excepting patent laws so that die shops able to convert some equipment to make GM/Ford/Chrysler but not currently under contract could be able to start doing this, or new shops spring up to cover the demand. That solution, or for existing parts facilities being given "mini-bailouts" if they need them until a buyer is found. Let the production of cars fail, but retain production of parts for existing/past cars until either reliant businesses transition to foreign cars, or to any new domestic automakers that might rise.
btw, I've learned quite a lot from this thread, thanks to you and other knowledgeable posters.