If the "Classic Modernization" program had been implemented in the 1980's, it might have had some success.
However, as stated above, the 747-100 and 747-200 (and even the -300!) are on the way out. Most are completely phased out of passenger service, and within the next few years the only examples will be flying cargo.
With the virtual phase-out of MD
-11's in pax service, the supply of newer cargo airplanes - especially with two-person crews - has increased dramatically. The expense of conversion is too much when a newer airplane is ready to go.
Let us also consider that for many airlines, the 747 is simply too much airplane. The 777 was originally conceived and designed as a DC-10 replacement. Improvements have made it the large-size long-haul airplane of choice. Interestingly, some of the earliest 777's are a bit underpowered and, in my opinion, should be the airplanes that are "modernized". United could get a lot of mileage out of the early models with just a few upgrades.
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