Nix was sounding pretty neat until:
"There isn't anything like apt which solves a SAT problem in order to satisfy dependencies with lower and upper bounds on versions. There's no need for this because all the dependencies are static: if a derivation X depends on a derivation Y, then it always depends on it. A version of X which depended on Z would be a different derivation."
That's nice and all but have you considered: I don't want 10 versions of a dependency on my system