What a mismatch of goals. The Steam Deck is meant for games and the steam platform tries to provide a single unified system library that games play under. Nix is all about creating a new custom system library environment for every single application.
Well that's what Steam does too. Creates a whole new Wine prefix with specific tweaks for this specific game to make it work. Pretty similar if you ask me.
Nix is a great fit for the Steam Deck because it lets you install additional software safely, with no risk of mucking up the base system, and because it has a HUGE set of packages, so most things you might want are already going to be there.
