Thermostat
Hi
You could have added that the PF may have lacked experience of HAVING to Hand-fly at high altitude and high Mach. ( Not normally allowed under RVSM, where Autopilot is mandatory.)
Jacques Roysay covered aspects of Stall Warning in Cruise at High Mach Number and High Altitude in AB's Safety First in Jan 2011.
An afterthought : there must be some combination of times and areas or routes where this strict rule could be relaxed by agreement with ATC. Watching radar traces of some cruising (civil ?) aircraft occasionally zigzagging along an otherwise empty airspace area, where we might have requested a "Direct to..." two or more check points further on, towards our destination. Even then, in the 1960s+, every mile saved was worthwhile in fuel and flight time.