The poster's confused, but I'm not sure it's necessarily true that the city of Detroit should have a value that's greater than zero. If the current cost of paying pensions and the cost of maintaining (underused) infrastructure is much larger than the expected tax base, than the value of the city (not including private property) could well be negative.

That said, it's unlikely that's true, because there's still a lot of option value available if someone's willing to make hard decisions and/or shed some costs through bankruptcy. (But it's going to be really unpopular, like not paying pensions or simply shutting down city services to certain neighborhoods.)

America doesn't really have a model for shrinking city infrastructure. Detroit is the first test of what happens when a major modern city stops growing and starts shrinking in a long-term way. It's hard to scale down government and infrastructure in a smooth way.