"Bob Gardner" wrote
An aspect of max altitude that has not been mentioned here is the
"coffin corner" where Mach buffet and stall speed come together.
Nothing to do with engine failure, but a consideration when flying
high.

True..and "coffin corner" is weight dependant.
The 41,000' limit for the B-707 was determined by
the time required to execute an emergency descent
to 14,000'.

Bob Moore