Given the obvious gradient of atmospheric pressure, mach 3 at 24 km probably equates to a much higher speed at 100 km...

It's not just a factor of 4. The atmosphere scales by a pressure factor of e per scale height (~10-11 km from memory), so from 24 to 100 km is ~8 scale heights so pressure is e^-8, or ~10-4 at 100 km, allowing a speed of 10,00 times as fast for the same heating rate. (Since the speed of sound also varies, this is NOT MACH 30,00, but it's still fast!)

Everything depends on the entry profile and the drag distribution vs altitude. Peak heating depends on entry speed, altitude, and allowable g (permitting more g paradoxically alowws less peak heating, for perverse mathematical & physical reasons)

Plus, the Scaled composites craft is designed to last only 3, or 5, or 10 descents, then be thrown away. The Shuttle is supposed to last 100+ re-entries - essentially indefinitely...

(so NOTHING can wear out, or it crashes and burns, as we have noticed, of late)