Steam Deck

I hope so, but only if the option to install other OS's remains an easy option. I love android but installing a different operating system on my phone is so much of a pain in the dick that it's not even worth it.

I feel like I'd probably avoid a handheld if the option to install windows wasn't there, even if I don't end up using it much.

If my choice was a default windows install with the option to install steamOS myself, or a default steamOS install with no other options, I'm choosing the windows install every time.

Only possible if all handheld machines have transparent hardware designs, i.e., all electronic components inside are known, have open source drivers and do not rely on third-party proprietary drivers or reverse engineering. This is due to Linux itself rely heavily on open source software and doesn't play well with proprietary parts (take Nvidia GPU for example, every person who has it in their Linux machine knows it causes headache once in a while). Unfortunately, so far only Valve's Steam Deck has a hardware specs that satisfy this requirement. The other ones more or less suffer from closed source components

Looking at how many celebrated ROG Ally shipping with windows I doubt it will catch on. Only possibility I see is if valve would do profit sharing with the handheld maker for purchases made in the steam store.

For a third party to ship with steam os now would essentially mean they are also supporting the largest player in the market with no gain for themselves.

No, Steam doesn't support arm. So my hacked Switch runs cobbled together emulationstation + xfce + antimicrox + onboard. I don't like Horizon OS and only SuperTuxKart works flawlessly on Android for me. In SuperTux, the sound desyncs from the game. Minetest doesn't support gamepads, and I couldn't find any Android alternative to AntiMircoX. I also just don't like how 99% of Android apps "need" Google Play Services.

Linux is what I need, but there isn't any decent interface that isn't SteamOS (x86 only) or RetroArch (everything must use libretro) or batocera.linux (their version of emulationstation completely shits itself when ran outside of batocera, and I really don't want to recompile batocera)

No. Steamos is only really great on deck because of the whole making the hardware and software thing. If other people use it it loses that and you end up with a computer with a less compatible OS.