Linux and Windows both already support secure boot. Anyone is free to make a locked down version of Linux, and they have (game consoles, Android). Desktop Linux has way less market share than Windows. So what would you like SteamOS or the Linux kernel to do? I'm not fond of this stuff, but at least if people are using a partially locked down video game focused Linux, it's better than them going to Windows.

My comment is about remote attestation, which is incredibly user-hostile technology; not secure boot. Secure boot enables remote attestation, but by itself is mostly harmless.