United unofficially calls their 777s "money savers". Why? Terribly cheap to maintain, very efficient fuel burn, large seating capacity.
Fragmentation, or the tendency for airlines to fly smaller equipment with more frequency, is still in vogue. Why fly 1 full 744 and another 2/3 full 744 to Tokyo when you can fill 1 full 744 and 1 full 777?
When UA cut its schedule dramatically, of course they were also going to trim equipment capacity dramatically. Right now there are anywhere from 12-14 747-400s sitting inactive.
Don't worry--they'll get called back soon when United announces schedule additions.
So far, here are some (if not all) the routes where 777s have 'replaced' 744 service:
JFK-NRT: 1x daily
ORD-BJS: 1x daily (temporarily suspended soon) -- replaced SFO-BJS 744 service
SFO-NRT: 1x daily
SEA-NRT-BKK: 1x daily
* ORD-NRT: 1x daily (this 2nd daily flight was cancelled after 9/11 and now only 1 744 remains)
Rumoured: LAX-AKL and SFO-TPE
Notice how not THAT many 744s have been displaced.
Also, last year we saw some 744 displacing 777 equipment to Europe. While the 744 is a regular on the ORD-FRA run, and occassionally from IAD-LHR and IAD-FRA, the 744 was regularly scheduled on SFO-LHR, IAD-LHR, and IAD-FRA last summer.
There is no question that UA has been wanting to replace 747-400s with 777-derivatives for quite some time; rumours about this have spread throughout the company for more than a year. The only thing left to factor in the decision is: a) time, and b) -more importantly- money.
I think if Boeing can offload UA's 26 oldest 747-400s (leaving them with 18 newer ones), those would help to finance 777 replacements.
UA still needs 747-400s no doubt, just not 44.