I’ll confess. In the last 40 years I normally started my O2 above 12,500. I estimate I have something like 3-4000 hours between 5000-12500 without O2. I never felt a need to start any lower. I must be brain dead by now, which explains a lot
But seriously, each individual is different. Not everyone needs O2 at 5000 feet, or even at 10,000 feet, and it does not make them irresponsible idiots if they don’t stick a cannula in their nose every flight at every altitude.
If I ever crash please don’t blame it on hypoxia.
Just wanted to offer a different perspective.

Ramy