The way I see it, the 777s will not replace the 747s entirely. The 777, whether it be a -200, -200ER, -300, or -300ER, will replace most older 747 models, as Raggi explained.
With United Airline's point, I want to justify that. Yes, say the 747 prototype first flew on 02/09/1969... some airlines took delivery of their 747-100s and -200s in the mid 1970's and even into the 1980's. 747SP deliveries took place from approximately 1976-1984 (not sure on exact dates).
The 777s now will replace the oldest of the 747 fleet an airline currently operates. It might be lost profit in number of seats on the plane, but in terms of range, fuel, and crew members, there is a significant amount of savings there.
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