It was discussed in the Debian mailing list long ago, when Nix was not so polished [1].

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