I don't really get the idea of needing converts when nixpkgs is basically the largest and most up to date.. Using another Linux as a system engineer you would certainly know how to build outside the package system because you would have to.
Other people might be getting different benefits.
(Really this is just the configuration file vs language conflict, again. Oh, this is too complicated, make a config file, oh my config file can't express this special case lets add language features, why am I using a config language instead of a read language, oh this real language is too complicated...)
The end goal is that you have a reproducible, immutable build contained in some git-diffed config files which is all I want.
And it becomes especially nice when you have a single repo that configures all the machines you use, from your macOS laptop to your remote servers, with common config/programs shared between them.
Whether anyone else offers a nix/flake/HM version of their software barely moves the needle on utility of this setup.