A simple google search will show that there is a wide body of academic ( I know thats not your thing Dick) support for setting 10,000 ft as a limit for
non-assisted flight. Its supported by experimentation. And regulators have to publish a number. Pilots are expected to comply with regulations for pretty obvious reasons.
Personally I have been to high altitude in Nepal and seen fellow trekkers start falling over at as low as 12,000 ft.
And they don’t realise its happening ! Thats the point. Don’t self regulate. Just follow the rules.