Are all those packages available in binary format? Not familiar with Nix but that's certainly not the case for Arch. Arch has 85k packages in the AUR as source recipes but not as binaries.
I still think Debian makes a better use case for a server since it provides everything as binaries.
If you're going to use binaries what's the point of using Nix anyway? The declarative aspect is nice in an abstract sort of way but you can achieve a system deploy or restore just as fast by installing a vanilla system and a few config files.