American couldn't make ORD work to Asia. I would have to bet that they have much more connecting passenger traffic there compared to Alaska in Seattle. The big difference is that no one in the Pacific Northwest will want to connect backwards to ORD or DFW. Is Seattle that large with local demand and Alaska Air feed to make it work compared to Delta and the Int'l carriers from Asia that already serve Seattle?
There will also be some connecting feed from AS destinations in the Mountain West and Midwest in addition to connecting feed from AA hubs and the Pacific Northwest on AA's international flights out of SEA.
AA's codeshare partner CZ has a hub at PVG, and AA will also be able to offer connections from its SEA-PVG nonstop flights onto CZ flights out of PVG to destinations in China, Taiwan, Thailand, and Vietnam. In addition to connections onto CZ flights out of PVG, AA will also be able to offer connections from its SEA-PVG nonstop flights onto MH PVG-BKI/KUL and UL PVG-CMB flights.
AA can likely make SEA-PVG nonstop service work with connections to/from China, Malaysia, Sri Lanka, Taiwan, Thailand, Vietnam on CZ, MH, and UL on the PVG end, connections to/from the Pacific Northwest, Mountain West, and Midwest on AS on the SEA end, connections to/from AA hubs on the SEA end, and SEA-PVG O&D traffic.