If SteamOS devices become popular enough, Linux for gaming becomes popular enough, therefore more games will be made for Linux and more people will finally find a way to abandon Windows forever.
The compatibility stack Steam has set up to run games also allows stuff like Photoshop to run, so that's also becoming less and less of a reason to remain on Windows.
1. Corporate users on Windows, because the computer salesman bribed the person responsible for purchases at corporate to buy PCs with Windows. These users don't want to even touch a computer when they're not at work. Rather they'll use their phones or maybe an iPad for personal computing.
2. Gamers on Windows, because they didn't want to buy a gaming console (such as the Steam Deck).
3. Mac users. You see them everywhere in coffee shops. They're usually self employed or working remotely.
The other category of users are so small as to be marginal.
You write that you can only think of games, some productivity software and legacy enterprise software as the reason for why your OS would matter. That sounds like you're a Linux user? Linux users have been starved of having quality software for so long, that they've grown accustomed to using web services, even though these are second rate. Anybody else I've met wants to use native apps.
In my perspective, it doesn't make sense that somebody would use a web application for things such as e-mail or calendars or RSS. Why torture yourself with a bad experience, when you can use state of the art native apps for almost anything on MacOS?
Or a windows user. Like myself as well.
Steam OS, at least in the form that exists on my Deck, is not suitable for general purpose. True, it has a desktop environment, but it's "jailed" - of course with an option to jail break it, but anything you do outside of the user space is subject to being wiped the next time you get an OTA update.
Then again, OTA updates are necessarily only a thing on the Deck and I suppose this wouldn't be a problem on the Steam OS distribution available for installing on other hardware.
You mean outside of /home? This is actually excellent. Rock solid base OS that can be A/B booted, deliver extra apps via Flatpak/AppImage - this is close to what macOS achieved, minus all the crap Apple has been pulling recently. I'm tempted.
More precisely it would be /home and /var. Persistant changes can in theory also be done with systemd-sysext, but it isn't exactly the most user friendly thing currenty.
https://blogs.igalia.com/berto/2022/09/13/adding-software-to...
There is a reason azure is their cash cow nowadays.