They've said elsewhere (sorry, don't remember off the top of my head) that SEA is basically at max capacity without more gates (they've been being added... slowly) and the international arrivals facility being complete. After the IAF is built, I would expect SEA will grow further. SEA, strategically, is closer to Asia than LAX is. An hour and a half closer to NRT, ICN.... that is a competitive advantage vs AA and UA flying it from LAX/SFO, and with the huge Amazon expansion and growth in SEA, it makes sense to compete in SEA in a big way.