Competing against Northwest in their hub cities is a crap shoot at best. They will undercut you and flood the market with flights no matter how much they lose just to ensure you don't make money on those routes. Not worth it when there are opportunities elsewhere.
You've just explained one of the many reasons why so many business travellers in Michigan loathe NWA. SWA has only had 15-18 daily departures from DTW since the move from Detroit City Airport over 15 years ago. The standing reason for no increase in service has been unavilable gates, and departure slots hoarded by NWA. While NWA has reduced departures, increased fares, and used smaller planes, the majority of flights departing DTW are at near capacity. A new terminal makes the timing perfect for SWA to add 1-2 new destinations from DTW. LAS and PVD seem to be logical choices.
DTW has become one of the newest airports in the US, and also one of the least competitive. Comments anyone?