NixOS could have been good alternative as well and probably easier to maintain / develop, but I'm sure Arch as base takes less space than NixOS.
It would've probably lead to massive confusion due to users not getting the "nix-way" and it seems like easy hack-ability is part of the steam-decks value proposition.
The Steamdeck modifications often get broken in updates, because the Steamdeck software is an image that is managed by Valve. Meanwhile, with Nix I think it would be easier to maintain modifications, as you could layer them on top of each other.
Also, most Steamdeck users:
- Don’t modify their Steamdeck
- Use Flatpak, which works on Nix
- Run an install script from the internet, such as DeckyLoader, Emudeck. Which often break with updates.