I can't remember how many flights NW has had historically on MSP-SFO in summers past, but right now for the summer it is showing 7x daily, 6 757s and 1 A320.
I'm not expecting much of a retalitory response, it's once a day for connecting feed that they carry already for the most part. A few years back AA added MSP-MIA quietly for its morning LatAm bank, and NW didn't retaliate. Now I just wish they added a second late flight out and early flight back for the deep S. Am. bank...