SteamOS is not limited to playing games distributed on Steam. It's a Desktop PC running an optimized interface by default. You are always just three button presses away from launching Desktop Mode and installing whatever you want, as long as its either made for Linux or runs using a compatibility layer.
I'd even argue that a device like the Steam Deck, with its open approach in almost every design choice, is a counterexample for "dumbed down devices". It has an easy-to-use UI for people who don't want to fiddle around with it, sure. But as even its operating system is, for the most part, open source, you have many more possibilities than you have on a proprietary platform like Windows if you want to tinker around with it.
Handling input from the gamepad and having a massive library of community contributed controller configs is a massively underrated selling point, imo.
A Linux based desktop is often quite literally better as a real computer for regular people to get things done with than a Windows machine.
It's kind of a weird horseshoe: Obviously for a lot of power users Linux is better, but I'd also argue that Linux is a much better proverbial "grandma" operating system as well; she can do A LOT with simply a working browser.
Windows just really became a mess UI-wise, not only in its settings menu but it is bleeding to their user-facing stuff. The task bar is a mess, the notifications overlay is a mess, the startup menu is a mess, the copilot integration is a clusterf* if the user doesn't know what copilot is, the sharepoint integration is a mess, the popups to create and sign in with windows are a mess, all the useless microsoft built-in apps get in the way. Not to mention all the crapware OEMs add to the mix. Many of these problems also plague MacOS (and Ubuntu!)
Windows is going to die because most people don't need all this crap, they need to double click an application (usually the browser) and just run it. Maybe they need to task switch being applications and do some file management sometimes.
The enterprise and home operating system business as a whole is going to die, likely taking mobile OSes with them if anti-competitive legislation catches up. There is no money to be made. Big companies are sponsoring open-source Linux improvements that trickle down to consumer space for free.
Who knows, it is quite possible proper desktop applications like games will just start being shipped as bundled microkernels and loaded in a virtualised environment in whatever platform, or web assembly really takes off and we start seeing wasm-first apps being distributed outside the browser. It seems we are heading towards this direction of platform agnostic applications.
That's the pitch? Some games that run fine on Windows 'might' work acceptably on SteamOS?
SteamOS doesn't do anything besides play games distributed on Steam. There's absolutely no benefit for a Windows OEM to drop it for an OS that has < 1% market share.
Not everybody has the luxury of purchasing different computing devices for different use cases. An ASUS ROG will play games (Steam + everything else), but it's also a real computer you can write programs on or school papers with.
We need to reverse the tide of electronic products getting dumbed down every year in the name of "user experience".