I am a (big) Delta fan, but I must bow to this big strategic move by AS and AA. This commands respect. I must say that I have started worrying for Delta in Seattle and Portland over the past days.
I obviously don't have any data to support my reasoning but I will give my two
(please take this with a grain of salt):
I think it would be best for Delta to add more U.S. and international destinations from Seattle and Portland. Delta went this far to create a hub in SEA (even if it is still a distant second place to AS). It would be a shame to give up. I understand there is barely enough space left to grow, but I can't imagine this could be already the end of the expansion plans for DL.
It certainly won't be a creative idea, but DL may copy AA to add a SEA-BLR and SEA-LHR to capture corporate contracts, and to limit the advance of AA.
For internal flights, DL may add SEA-MIA, SEA-DFW, and SEA-CLT.
Yes, there are OneWorld partners flying to Seattle, but so does SkyTeam partners: Air France, Virgin Atlantic, and Korean Air. DL may rely to these JV in place to fight back AA.
Last but not least, I think DL shouldn't underestimate the importance of Portland, and start expanding more, to give some reasons for AS frequent flyers based in Portland to jump ship.
Not everything happens in Seattle in this "battle". I would love to see DL adding the following: PDX-MCO, PDX-SFO, PDX-LAS, PDX-SJC, PDX-YVR, and PDX-CDG.
It is all wishful thinking of course, but I hope DL will never give up any efforts they have made over the past years to operate from the North-West corner of the U.S.