Pace, the last line in your post is open to question. You say
"With commercial pilots any decline in performance would easily be picked up under pressure and mental loading in a simulator....!"
Then WHY do I read that the bean counters are cutting down on the simulator sessions, that these sessions fail to cover some of the most important items such as recovering from a stall at 38,000 feet over the Atlantic Ocean, etc etc
and that first officers are still too deferential in certain cultures?
How many simulator sessions does the average flight instructor in light aircraft afford a year? I bet not many. Gliding instructors in the UK are tested at least once a year in a real life session with our CFI (Chief Flying Instructor in UK parlance). How is the hard working professional flight instructor monitored these days?
How often are you monitored, Pace? fess up!
I still think it is up to each pilot with the responsibility of somebody's life in their hands to realise when it is the only decent thing to step down. Just as it still is my responsibility to speak up when I observe something happening that could cause an accident...even if the Pilot In Charge of the Flight thinks I'm a pain in the butt.