Now let's not throw out the baby with the bathwater here....
It's not the airframes, it's about engines and fuel.
The 747-400 airframe still has 20 years of life in it. Why throw it away when the alternatives don't offer a huge leap in efficiency.
My guess is that fuel prices will moderate once alternative energy picks up a lot of the demand for electricity. Perhaps we should be looking at new power plants and synthetic avgas.