I genuinely feel like there's an appriciable percentage of the population that don't even know that other operating systems exist. For whom Windows is "the computer" and for whom even Apple being a separate operating system is a difficult concept to grasp. If that's truely the state technical literacy is in with a sizable slice of the population, then it's quite the hurtle even explaining the basics of what Linux is. Let alone using it.

Computers used to work this way.

You could even ship the computer with the USB stick pre-installed.

And this wouldn’t be impossible with Apple hardware; it has a bootloader built in that can boot from any functional and signed OS; could be Apple supplied, or something like Asahi. Or, with such a rule in place, they may also be required to not get in the way of installing other OSes and have to fully document the boot process and driver registration process, preventing signature-based lockdown completely.

Well 100% in fact as that's NOT the definition of "out of the box".