I prefer to isolate my development environment already in various ways (virtualenv, containers or VM depending on the project) so I don't need that parts of NixOS. My systems are already run on a well-curated set of software. Two decades allowed me to fine tune that aspect pretty well.

While I understand the gravitas of NixOS, that modus operandi just is not for me. I'm happy and fine with my traditional way.

However, as I said, I understand and respect who use NixOS. I just don't share the same perspective and ideas. Hope it never breaks on you.