Good luck locking loose mainboards sold for the DIY market, which don’t come with anything installed by default, to a given OS, the only way that could maybe work is forcing the OS in ROM.
Another way would be to discontinue the socketed desktop form factors and replace them all with mini PCs that are as locked down as the current Macs.
Good thing PCs aren’t locked into Windows.
I’ve been happily running the mesa-dev stack (mesa-tkg-git from the chaotic-aur repo) both on semi-current hardware (an RX 6600 that’s sidelined by a bad fan atm) and somewhat older hardware (the Vega 56 I’m using as a backup because it’s my second best card after the RX 6600) for a while now so I don’t know what you’re doing.
So you’re suggesting MS will somehow block non-Windows OSes from installing, even on hardware like loose mainboards for building your own PC with, or even on barebones mini PC kits or certain laptop SKUs, which don’t ship with an OS installed to begin with and expect the user to install it themselves? I mean, unless something extreme happens like changing the entire PC platform to be like the current Macs, that won’t be feasible.
Also, doing that would kill the Steam Deck which I doubt Valve would take sitting down.