I think you have to view Delta position in Seattle as trying to compete hard on TPAC/TATL and to Hawaii. It's why they serve all 4 Hawaii airports out of SEA and KIX/PEK/ICN/NRT. Would flying PHL-SEA allow passengers to more effectively fly TPAC/Hawaii with Delta? No it wouldn't. They can connect through DTW/ATL and spend more time on a long haul aircraft.