I think UA didnt succeed because they are the same alliance as NH and didnt have many onward connections to other parts of Asia from NRT.
The initial loads for NH on the SEA-NRT route when they started it were super light, like less than 50 pax in Y!
Now the NH flights are almost always high load.
JL would do much better as the only Oneworld partner and with plenty of connections in Asia.
Frankly, i am quite surprised there is up to now, still no Oneworld partner to Asia out of SEA.