If they aren’t able to work out a deal to consolidate with surrounding counties then the city should consider internal consolidation. Consolidate the citizenry in the middle and “abandon” the outskirts. Turn them into public parks or something similar. It would be the death knell of Detroit as a major metropolitan city, but it might give them a chance to rebuild.
I don’t know if this is advisable or even possible, but I think if the city works with its residents it could still make a go of it.