SWA only has about 21 daily flights at Detroit Metro--11 to MDW, and a few apiece to BNA, STL, and one to Phoenix. That's hardly a "crapload" by WN standards. The airline has more flights at Providence (28) and almost as many at Manchester (18) as compared to DTW. Cleveland is at about 25. Contrast that to BWI which has over 120.
Scott's right--WN will probably build up at non-fortress airports first. What so many of the Detroit folks who kiss Northwest's Emerald-encrusted ring don't understand is that within a few trips on Southwest they'd save enough money to buy a roundtrip on high-fare NW to most destinations in the US anyway. But as Tony Soprano would say, whaddya gonna do? People who don't want to be helped can't be helped.
Need a new airline paint scheme? Better call Saul! (Bass that is)