> there has simply been no incentive to improve flight times It's more that you just run in to real limits of physics; many planes already fly at mach 0.8 to 0

> there has simply been no incentive to improve flight times It's more that you just run in to real limits of physics; many planes already fly at mach 0.8 to 0.9, and going faster than sound is always going to be difficult because of the sonic boom. Realistically, there just isn't that much you can do without solving that first, which is a difficult problem to solve in a practical way. And even if you do, the fuel efficiency is always going to make it much more expensive.