DB - while you have a human in the cockpit who can choose to ignore procedures or forget checks or make cognitive errors, you will continue to have accidents.
Is it possible to select, train and ensure competency of pilots to the point where they don't make mistakes? Possibly, but how much will that cost and who will pay for it?
At the moment, just enough seems good enough for the employers, customers and regulators. As long as you have the perception of safety then people will keep getting on the aircraft and, after all, it is statistically much, much safer than riding a bicycle through London.
I am not saying we shouldn't keep on trying to minimise accidents since the same lessons need repeating constantly - like 'fly the aircraft' but if you don't accept the fundamental premise that the human in the cockpit will always be a weak link then you are likely to put all your effort into one direction. Concentrating on autopilot training for example, you might eradicate all bad practices in that area and then be horrified when someone manages to find a completely different way of crashing the aircraft.
The biggest error that keeps being made in flight safety is to identify a reason for the human error (in your example a tough session in the gym) and create a solution (no more pilots to use the gym before flying) to prevent that particular set of holes in the cheese lining up again (as they were never likely to do).
If you select your pilots well, train them well, motivate them well, vary their tasks to avoid the mundane and repetitive and ensure that they are not pressured to push on or take risks to get the job done - then you might have a chance of achieving aviation Nirvana but it will remain a Utopian dream because the realities of cost and competition will always drag everyone down to the lowest common denominator.