The success or failure of an airport, especially a hub, says very little about its city. ATL is the world's busiest airport but Atlanta is far from being the biggest city.
My usual itinerary is TLV-JFK-LAS-ATL-JFK-TLV yet I don't live in any of those cities. I am about 200 kilometers from TLV and fly out of it because it is Israel's only airport. I fly into JFK in order to connect and go to ATL to visit some friends.
It would be interesting to see how many people flying into/out of DTW actually live in Detroit. I would imagine that a large number are from affluent suburbs and an even greater number are only connecting.