While it is true that for this decade UA will be better off buying more 777-200ER's and start buying the 777-300ER, they ARE going to need something larger than a 747-400 once those planes reach their retirement age after 2013.
Remember, the number of passengers continues to grow on international flights, and airport landing slot capacity is definitely NOT increasing at the majority of Australasian airports unless massive amounts of money to expand airport capacity is spent NOW. Even now, the 747-400's UA flies on routes to NRT, SEL, HKG, PEK, and PVG are running at capacity, and UA's trans-Atlantic flights to LHR are also running at capacity. Flying more frequencies is NOT going to work in the long run because of the landing slot limitation issue.
Henceforth, while UA's 777 fleet will work wonders for long flights from smaller population centers to Asia and Europe, for flights from JFK, ORD, DEN, LAX and SFO, there will be a need for a larger plane, like it or not. UA could fill this need with the Airbus A380-800, or wait for Boeing to develop the Blended Wing Body (BWB) flying wing capable of seating 800+ passengers on a three-class configuration.