NW and UA fly daily SEA-NRT flights (NW is a 742, don't know about UA), while AA flies MIA-SEA-NRT daily. When MIA-NRT goes non-stop (I think it's going to happen within three years), I don't know wether they would stop SEA-NRT, but I don't see why they would.
I know NWA and AA fly to Tokyo from Seattle. I know UAL flew Seattle to Tokyo but not sure if they still do. The Seattle to Tokyo market is well served but not by JAL. The reason why I bring it up is my father does business with the Japanese. They fly over all the time for meetings on one of the American carriers and not one of them has had something good to say about the US airlines. They always say that if JAL or ANA served Seattle that would be there first choice. This leads me to believe that JAL or ANA might be missing out. Many Japanese tourist would most likely prefer to fly one of their own airlines.