> Their bootloader explicitly supports other OSes

That’s true but that’s probably only so that it wouldn’t have been a subject when Apple Silicon Macs were released because Intel Macs weren’t locked.

In reality, the bootloader isn’t closed (yet) but the hardware is so much undocumented that it’s easy to understand that Apple doesn’t want anything else than their OS on your mac. The « alternative os » situation is actually worse than it used to be with Intel Macs and Apple is paying a lot of attention in never talking about this "feature".

IMO, they will just quietly remove this possibility on new generations when everyone will have forgotten that boot camp used to be a thing.

Eh, you may be right, but there's a big difference between "they are going to forbid other OSes by placing a software restriction where they explicitly permit things now" and "they already effectively forbid other OSes by not publishing developer documentation for proprietary hardware"--that's a tall order, and not a bar that many other hardware manufacturers meet either.

Like, could they lock down the bootloader? Sure. But that's effort they'd have to put in for minimal benefit at the moment. Opening up their hardware would be a lot more effort for questionable benefits (to Apple).