Migrating Debian to nix might be possible, if that was desirable. You could have a compatibility layer for a while, and once everything is using the compatibility layer (in about 200 decades?), you deprecate the old package management details.
Honestly, I think it's a migration really worth it. Nix (and Guix) are quite mature now. The advantages they bring into the table are massive.
The whole Debian ecosystem would become a lot more integrated and robust. It would be possible to develop packages at their own pace, without having to keep all dependencies in sync with the whole package tree. Besides, no more dist-upgrade breaking your whole system. It would look a lot like a rolling release, but with none of its disadvantages.
It would be also possible to turn all Debian flavours into little declarative Nix blurbs. There are countless advantages.
[1] https://lists.debian.org/debian-devel/2013/02/msg00374.html
But I also really appreciate the massive effort that Debian maintainers make, and the sheer number of those maintainers.
Combining whatever human processes Debian have in place to keep that going with Nix would be fantastic. Right now, to use Nix regularly, you really have to be willing to read a lot of Nixpkgs source code.
Edit: I should also note that I do actually currently use Nix on top of Debian for my work machine. Servers are all NixOS machines deployed with NixOps though.
1. Too few marketing. I only heard of Nix for the first time last year.
2. When you boot a virtual server, the UI usually offers you Debian, Ubuntu and Centos/RHEL images, sometimes Suse/SLES or Arch. So again, less visibility, and if the hoster does not allow you to bring your own image, it's a pain in the ass to install anything else.
3. (Going off topic for a second: I probably would have switched to Nix(OS) already if it wouldn't entail effectively abandoning the configuration management tool that I maintain.)