"How do you zero the time frame?"
Easy: purchase 747SP. Take cash equivalent to purchase price of 777 and spend on repairs/upgrades to 747SP. Voila
... a super cool 20 year old airliner, that all the 20 year old airline enthusiasts really like. And the operating/acquisition costs are only 25-50% higher than 777!
[remove tongue from cheek]
I would love to see this happen. But airlines are pretty good at figuring out which planes will make them money. The market has voted on the 747SP and the answer is a resounding no.