Android devices are unlocked, non-manufacturer OSes are still well outside of "easy" to install, much less get all features working.
The main problem is that people maintaining the OS build for your hardware is the same people making the hardware. If legislation forced to divest both into separate companies the support would last far, far longer.
I would also want that for TV software or car software.
But I guess there is no money to be made on that software...otherwise we would already see a bunch of non-manufacturer companies offering software for 3rd party devices that are unlocked, wouldn't we?
Or maybe the used hardware is just not documented well enough
<sigh> If only that were true. I have a Samsung S20 with 128gb storage and a adequate battery that is ewaste because of the locked bootloader and now permanently vulnerable kernel.
Not disagreeing with your premise (the same should be done with ISPs — own the connection or the wires, but not both). I'd just take the "hard" install with fewer features over what we have now.
The main problem is that people maintaining the OS build for your hardware is the same people making the hardware. If legislation forced to divest both into separate companies the support would last far, far longer.