"..it is amazing overall that NW does generate the traffic required to fill the 747 on its transpacific routes but out of MSP there is no need for it."
Yeah, right...that's why NW flies routes like JFK-NRT, DTW-NGO, and believe it or not, MSP-NRT at extremely high load factors. This is gonna sound INCREDIBLE (hint of sarcasm) to you, but NW is going double daily MSP-NRT in April, with 747-200s. Over 2,000,000 people a month fly Northwest's international routes to countries in the Pacific.
I might agree with you that MSP alone cannot sustain NRT service, but you're leaving out connections at MSP from people all over the Upper Midwest that want to go to Asia. Many of NW's passengers on MSP-NRT come from cities like Madison, Milwaukee, Omaha, and Des Moines, WorldPerks members that choose to connect in MSP because it's MUCH more convenient than O'Hare.
And the last part of your statement "DL knows it can't do it out of ATL, AA knows it can't support it out of DFW" regarding 747 service, of course AA and DL can't fly 747s out of their hubs to Japan, or anywhere else for that matter, because they don't own them...
One more thing that I might add...NW's presence in Asia definitely helps them fill aircraft in both directions. WorldPerks was voted "Best Frequent Flyer Program" by the readers of Time in Asia, and NW is a frequent choice by Asian travel organizations like JTB for tours and trips to places like Hawaii.
Create your own luck.