..."Folksinger John Denver former lead singer of The Mitchell Trio, back in the early 70s could sustain a high Bb with ease, and could hit a C without much difficulty".

Forgive my skepticism, but could you point to any recorded song in which John Denver achieved a high Bb , with or without ease, much less a high C ? If he had been able to hit anywhere near a high Bb he would surely have explored that upper reach in his collaboration with Placido Domingo ...IIRC the highest note he reaches there is a passing F#. Denver's highest "comfortable note" AFAIK was an E. Admittedly he had a voice which made it seem that he was singing higher than he actually was.   

Similarly, I have no recollection of hearing any Phil Ochs song in which he remotely approaches a high A ...I would be delighted to be pointed to a recording which proves otherwise, however.