It’s immutable (you can’t break the core OS, there is no deleting system32). You can’t install packages (like you would from AUR), but have access to flatpaks.
Firefox is preinstalled, but anything from flathub is also available.
So yes, it has all the things most people need from a desktop OS, and is harder to break, and is supported commercially.
It has a desktop mode, I’ve never looked into whether you can boot to desktop by default. But I would imagine if they released a desktop friendly version, that would be an option.
Back in october I travelled for a lan party. I didnt bring my linux desktop with me, and just brought my steam deck and dock, and when I got there, borrowed a keyboard/mouse/monitor.
Then i swapped it to desktop mode, and the people I was with all commented on “Oh wow! it’s just like a regular computer”
One of them has explicited said they were fed up with microsoft’s BS and would swap their gaming PC over to steamOS once it’s formally released for desktop (they were uninterested in Bazzite and wanted an official Valve release for their gaming PC).