It may be of interest to some that the early models of the B707 and DC8, those with turbojet engines (rather than turbofans) it was indeed possible to climb to such a high altitude that the wing would no longer be efficient, and the mach buffet could be encountered with some regularity.
This was well above the altitude mandated according to the aircraft weight versus cruise mach buffet onset, and led to early jet upsets...until the effect was clearly understood by line crews who had transistioned from four engine piston types.