Doesn't that also imply that it would be possible to fly a modified plane at the regular speed of a regular aircraft at that much higher altitude, and use way less fuel? Airlines' main cost driver is fuel, so it seems like they'd take advantage of that as much as possible.
If you go higher, the speed of sound decreases so you have to go slower to avoid going above the critical Mach number for your aircraft. Normal airliners have to stay quite a bit below Mach 1 to avoid any part of the airflow going supersonic, since that would create big issues like shock waves making the aircraft uncontrollable.
But due to the air being really thin, you also have to go faster. Otherwise your wings will not generate enough lift to keep flying.
