Ghost towns

What is the purpose of a city?  The Sultan of Knish's analysis of Detroit is interesting, but in some ways I prefer the take of his most recent commenter:

Cities fundamentally exist, and always have, to facilitate exchange among groups and individuals.  Geography plays an important part to where a city exists. 

Among other reasons, cities go away when either their geographical location loses its importance or when some important part of what was previously exchanged goes away or becomes much less important. 

Once established, city government exists primarily to improve the possibility of secure exchange.  That would include public safety and transport (roads and streets, wharves, warehousing, etc).  Anything beyond that is gravy. 

Once there is a reasonable service base of security and transport those who make regular, routine exchanges tend to move their families in, provided that the city is seen as "a good place to live." 

. . . Ancient trade cities have vanished, the American West is full of ghost towns based originally on mining, and the upper plains are emptying out due to having fewer farmers. 

When an important part of the echange goes away, the city had better find something to replace or go away, too.

Instead, Detroit seems to think it can survive as a P.O. box for the receipt of welfare checks.