IMO there are no "good" or "bad" markets. Any major metropolitan area with infrastructure to get people to the stadium could theoretically support a team. However, there are ownership groups that make good business decisions and bad business decisions. For example, I was skeptical about Philly due to the saturation of sports teams there. Their ownership group made some good decisions and were able to carve out some space. Columbus started off that way then HSG let things fall apart. It's hard to say what kind of owners a Detroit team would have.
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The placement of teams should be a strategic decision by the league to expand the footprint of the league. Unfortunately it appears to be an auction instead of planned growth. If a crazy billionaire in Fargo, North Dakota threw $75 million at the MLS Board, I think Fargo would get a team.
On the other hand, St Louis was a perfect location for a footprint expansion, along much the same grounds as Detroit (geographic more than anything else). But that ownership group turned out to be a disaster, MLS was smart to avoid them.