Lots of assumptions that could be totally wrong. Custom distros are not new operating system - majority of hw support is because linux supports it. It would probably be more work for Valve to support only their hw instead of keeping what linux/arch support.

I am also not sure why you think they wouldn't want to end up replacing Windows. Thats exactly what they've been trying to do since microsoft included their store in windows. That is more than a decade long plan in motion which already failed once but they are still at it.