you raise some interesting points.
One thing you may be forgetting is that not everyone on those flights to/from NRT is American. What I mean is that the flights from NRT-PEK/HKG etc can also fill up with Japanese and other Asian-nationalities (mostly Korean due to geography) as well.
I don't know about the market that NW has in Japan and NE Asia for those routes and whether it's actually more cost-effective to keep their ops as it is instead of making a city like SEA a sort of NW-East Asia mini-hub, but I agree, NRT is getting a bit too crowded with 4 hubs there.