SLF
The wing is rather specialized for the four-engine configuration, making it a twin is impossible now. The major issue is the four engines and associated fuel loads are designed to spread the wing bending moments.
Grounded27
No doubt the B747 fleet has vastly more flight hours and accumulated RPMs flown, close to the entire population of the Earth; however, the direction of sales is the indicator of the future. B777 sales will, without doubt, eclipse B747 sales and will eventually eclipse B747 RPMs as well. Except for dense city-pairs, the B777 will be the largest airliner for many years to come. Airbus admits as much with the A350 design, the low sales potential of the A380 and the B787 order book. Admittedly, Boeing has to deliver it and make the promises come true. The A380 is a cautionary tale, indeed.
BTW, any bets on the A380 delivering 1400 planes or having 800+ in service at once?
GF