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!
There seem to be some misconceptions and expectations from you. SteamOS is the Steam Deck operating system and will always be. So once it is available to the public, then it will always be the Steam Deck OS. That is what SteamOS is. It is build on the Steam user interface divided up into desktop mode and gaming mode.
At the current moment, there is no Steam Deck OS available to the public. The downloads from Valve to install on other machines is an old version called Steam OS 2, which is a completely different operating system not related to Steam Deck and abandoned for years in favor of the new Steam OS 3. Which is still not available. The stories and experimentation from other people you read are a community build operating system based on the recovery image of the Steam Deck, which only works on a handful of PC configurations that matches the Steam Decks hardware compatibility (in example no Nvidia).
So for short: Steam OS 3 is not available to the public yet, only an unofficial community created OS called Holo OS that is build from the Steam Decks recovery image.
And in general, Steam OS 3 works only limited as a general purpose operating system to replace a desktop. If that is your main goal, then I recommend to install a standard Linux system instead. But if your main goal is to play on the device with Steam, then Steam OS is perfect (once it is out). Also have in mind, a Linux based operating system will never replace Windows entirely, because many important programs you might use won’t work there. For some other programs you need to find alternatives and for some the alternatives might not be satisfying to you. It entirely depends on what you want to do with it.