which has always struck me as an accountant's idea
Well, it's really an engineer's idea. As engineers have improved the power and reliability of engines, airlines have been able to take advantage of that.
Which would you rather be on? A 777 with two modern, computer controlled and remote monitored engines or a 707 with four smokey turbojets with only 15% of the thrust and controlled by a hydromechanical unit using ten times the moving parts and serviced at whatever intervals?