SEA is very well connected to YVR, SFO and LAX and only provides hub services to AS, which doesn't really extend CX's reach beyond what AA can feed in through LAX.

If AS were to come in to OW that would make a difference, but it seems to me that PHX offers far more reach as a new US destination than SEA, and LAS would probably have far more O&D traffic, if there were a small enough bird with the legs for it.