I heard some time ago that, just with the technology advances made from Concorde, it would be possible to make an aircraft with half the weight but the same payload. Or the same weight and twice the payload.
The argument about the amount of energy is fallacious and wrong. Just because something has four times the kinetic energy (from travelling twice the speed) doesn't mean it uses four times as much fuel. In fact, the wing design of airliners is part of what stops them going faster - not the amount of energy input required.
Fact is that airliners run using about the same fuel per pax per km as cars - well, at least, the same order of magnitude. Work it out. There's a very interesting part of the Boeing website on the environmental impact of airliners.
Arthur C. Clarke wrote an interesting book called "Profiles of the Future", in which he comments on the two failures of futurologists - failure of the nerve, and failure of imagination. Believe it or not, at one stage people thought that anybody travelling more than 15 mph in a train would suffocate. The fact that people already travelled faster than that on horseback seemed to escape their notice.
At risk of losing one of my favourite tricks, guess how heavy a Concorde is at max AUW?
Now guess again.
The max certified is about the same as that of a B767-300.