A good pilot has a lot of spare mental capacity when loaded
That's true, but it depends on how you got there, in terms of both that actual flight, and in your flying history.
A good pilot will prepare the flight, and execute it, in a manner which ensures (as far as possible) that his cockpit workload never becomes unmanageable.
For example I can sure fly my TB20 by hand in IMC, and have had to do so plenty of times, even getting video footage of the failed autopilot behaviour
but I would not depart on a long flight with an INOP autopilot (unless I had a co-pilot) because an autopilot makes a ~10x difference to cockpit workload, and who should embark on a flight with both hands tied behind his back?
In the event of an autopilot failure enroute, one would not create workload just for the fun of it. One would not fly an NDB approach (in any kind of real near-minima wx) if there is an ILS 50nm away, and one would not fly a procedural ILS if one can get vectors. One would also tell ATC that in the event of a hold, could it please be above FLxxx (above the clouds), etc etc (helps with icing conditions too).
Every pilot will make more mistakes if pressed, and every pilot makes mistakes even if not pressed. I make some mistake on almost every flight. You hope that none of them are critical. Usually, with modern avionics, not one of them is
alone critical. He says... 170A tomorrow, no autopilot, no GPS, for the most part
At the other end of the scale, you need to be a very good instinctive pilot to be good at aerobatics, I would think (not something I know anything about).
And you get all sorts of people in between, because anybody who can dig up ~£8000 and can spare 50-60hrs or so can get a PPL.
The advantage which a high earner has (assuming he actually has time, which most of them don't until they are in their 50s) is that he is not short of money, and like it or not that is very relevant in flying, because currency, and particularly currency on type, is the most important single thing.
The disadvantage which a high earner has is that he will p1ss off most instructors