This would solve a major issue with SteamOS. The SteamOS itself is immutable/read only as you know. This means, one cannot install or make changes to the core system (it is possible, but that is temporarily until update will revert those changes back). That is why we have the Flatpak support. But many software are not available there (especially CLI tools, if you use them).
This is where the Nix package system comes into play. In short, it allows for installation of programs without touching the immutable part of SteamOS in a controlled and safely manner. These would persist even on SteamOS system updates. For a better explanation, have a look here: https://nixos.org/guides/how-nix-works.html
Valve confirmed: https://github.com/NixOS/nix/issues/7173#issuecomment-1603858003 that Nix package support will be integrated into SteamOS! That’s huge. In example I have tools which I cannot distribute easily for Steam Deck users, because Flatpak does not support CLI apps. Nix would solve this issue!
I don’t know what you mean by “steamdeck version”, but afaik the SteamOS you can download from steam is version 2 which is really old by now while the Steamdeck runs version 3 and is still not available for download outside of it. Mind you it isn’t just a major version change but they even changed the linux OS it’s based on, moving from Debian to Arch.
As for the replacement for desktop, the Steamdeck (and with it SteamOS 3) launched with full desktop mode available and while the core system is immutable as discussed, it’s still a fully functional desktop OS and as such it depends on your personal needs how much of a windows replacement it can be, just like any other linux OS. Also you might need a dock or other USB hub to attach a keyboard and mouse if you need them, but there is onscreen keyboard and touchpad on the steamdeck available.
