Additionally, if we are comparing the Linux gamer ecosystem to Apple, then also the Apple ecosystem should be taken into account.
SteamOS basically is a Linux Desktop. Steam Deck is just a hand-held form-factor PC with built-in Steam Controller, and SteamOS is basically just Valve's custom Arch Linux with KDE Plasma desktop and Steam "Big Picture" mode as "desktop" environments.
> "then so should Apple mobile devices."
No, they shouldn't, because Apple mobile devices don't run Linux, nor Steam.