I belive the problem is what is known as "Coffin´s coner" (sorry for the spelling)
With increase in alt the stallspeed increase, and the highspeed (Mcrit) decrease. At a point they will meet so you either fly to fast or to slow. I think the first Learjets had a problem with that, being able to fly very high. (I´m no Lear driver)
So one would have to fly supersonic, wich again means new engines, new intake design, new wings, new......
Another problem would be antennas. I belive to have heard that the Concorde is the only aircraft (comercial airliner) that does not carry TCAS, because the antennas would melt at high speed, and I would to expensive to develop new antennas for 8?? aircraft.