Domestically, just about everything NW offers out of SEA connects through MSP or DTW. With UA you're going through DEN or ORD, with DL your going through SLC, CVG or even DFW. AA hubs out of ORD or DFW and CO routes through IAH or EWR.
Although NW isn't usually my first choice, it would be nice to have some more non-stop options from SEA when flying to the east coast and down south. If NW opts to do that I think they could achieve some good loads out of SEA.