When you're programming in any old non-lazy language, your functions don't get automatically called, it's only when you put that call in code that's being executed. If you don't mention a thing in your main, that thing has no reason to get executed.
Nix is lazy because the whole point of the original work was "what if package management, but FP". The resulting /nix/store etc system has very little to do with FP and is, in fact, built by imperative bash scripts.
Depends, i imagine like most things - the answer is nuanced, or at the very least depends on the specific other decisions.
For example, i don't see why a configuration would install actual packages - if that's what you mean. Besides, i am not advocating against lazy by itself. I'm just saying that something which is both lazy and untyped is a huge PITA. Nickel adds typing, validation, etc - information, by which you can still have Lazy and a better DX by way of types. At least in theory, i've not used it much.
