There is a deal between antitrust regulators (would need to look up who exactly...) and Microsoft that secure boot on x86 must allow to install your own keys and thereby alternative operating systems. It does not cover any other architecture, so ARM devices can be and have been locked. Just Microsoft is not very relevant in the ARM space so it's less disturbing. (Others are locking ARM, too. But that's a different discussion.)

Also I would not trust on the current US administration to keep deals that limit US corporations in force forever.

I have used secure boot to secure Linux systems, so it is useful for Linux users. But the danger that oligopolists will misuse it is not unreal.

The US is not the only government with sway over US tech companies, and the OS market is now substantially more diverse than when secure boot on x86 was first introduced ~15 years ago.

Never say never, but I just don't see how the option for one to boot an alternative OS on the vast majority of commodity hardware is going to be rolled back any time in the foreseeable future.

it's like the antivax - we haven't had an outbreak of $x in $y years, why do we still need to be vaccinated?