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.