Now you’ll have a zillion users trying to install software in ways that violate all the assumptions that NixOS operates on, but which are still tightly coupled to your NixOS config. Now updates to your system, or even seemingly unrelated config changes (through some transitive dependency chain) can easily break that software.
So now we’ve basically removed half the advantages that motivate Nix/OS in the first place, and when stuff breaks it will look like it’s Nix’s fault, even if it isn’t.
On the other hand, nixpkgs is already the most comprehensive repository of system software out there, and for 99% of packages Nixifying it is pretty trivial. Hell, my NixOS config does that for 3 different GitHub repos right inline in my config.nix
It’s wildly under-taught. It explains like half of all problems in the world. Education: “teaching to the test.” Economics: optimizing GDP at the expense of non-material well-being. Maximizing shareholder value by selling out employees and enshittifying your product. Software: “data-driven decision making” optimizing short -term gains over long-term because they are more measurable. That’s just off the top of my head.