We already have head-to-head comparisons on a number of devices which show Windows 11 is 10-20% worse in gaming performance versus SteamOS. Not to mention that there's a lot less fiddling you have to do in SteamOS's gaming mode as opposed to the setup of a full fat Windows installation, which is a PITA to do on smaller screens. I think what Microsoft is doing with Game Pass is pretty neat, but beyond that they've fallen so far behind the competition that it may not even be worth trying any more. Especially if this thing debuts with like an $800 price tag.
What? Part of those "10-20% percent improvements" come down to how much better Linux as a whole has been optimized to be more efficient at resource usage than Windows, even more so considering how low the requirements to install the absolute bare minimum and even are (you used to be able to run the latest version of the kernel on an original Pentium before it was taken out for optimization reasons). A large chunk of Linux's system requirements beyond the kernel and terminal comes down to what Window Manager (i3, IceWM, Sway, etc.) and/or Desktop Environment (Gnome, KDE Plasma, XFCE, etc.) plus whatever software you run on top of that. Wine's Performance also is influenced by the fact it's a translation layer (which even if it doesn't cover everything accurately, lets it handle things better due to having a lot less bloat to sift through), and only uses the library calls the software needs instead of all of them at once like you would need to on Windows since a lot of the same (or more) libraries is the ones the entire OS runs on, execpt things like Adobe and MS office still don't work under base Wine or Proton, which people tend to use a lot besides gaming.
SteamOS and Proton didn't do any of that, Linux and Wine plus the code they made for Proton, and other things they didn't make like DXVK did. What they did beyond the changes made for Proton was rebrand Big Picture Mode, made it the default with some changes to make it more usable as a standalone enviornment, and put in default settings made to work with the specifications of what it is running on (Mainly the Steam Deck and the Handheld PCs it was "Officially" released on.) with an option to switch to KDE Plasma, which is going to be apartent when An offical standalone version of SteamOS does get released and the horde of people who've been hyped up as the thing that will replace Windows with no idea what Linux even is, the unwillingness to learn even the basics of it, and the Mt.Everest amounts of things different between Windows and Linux that will make you absolutely hate Linux if you don't know what they are and the sacrifices needed to make the switch is.
And if you consiter what it takes to run games through steam on Windows compared to Linux (Even on handheld PCs), how is running a web browser to install Steam and going through the motions of installing games and putting your prefered settings that much more complicated than pickng your linux distro, getting acclimated to how it functions if you never used Linux before or switched to a different distro (like going from Debian to Void Linux), and then installing steam the same way through a Web Browser or the terminal and all that more fiddling? It's even less of a non comparison if you consitered a Laptop or prebuilt PC with Windows preloaded vs a handheld PC like the Steam Deck where the process of getting Steam installed doesn't even take that much time if you actually know what you're doing because oh wow, you have to take a little bit to install something and configure things on the general purpose opperating system over one tailored closer to a specific niche. It's like saying the turtle is slower than the one that's been taught how to ride a wheelchair.