We have a good hunch. The basic theory behind Nix definitely goes in the right direction, and if we look away from all the surface-level nonsense going on in Nix, it's conceptually capable (e.g. [0]) of being a first-class language dependency manager.
For this to work at scale we'd need to overcome a couple of large problems though (in ascending order of complexity):
1. A better technical implementation of the model (working on it [1]).
2. A mindset shift to make people understand that "binary distribution" is not a goal, but a side-effect of a reasonable software addressing and caching model. Without this conceptual connection, everything is 10x harder (which is why e.g. Debian packaging is completely incomprehensible - their fundamental model is wrong).
3. A mindset shift to make people understand that their pet programming language is not actually a special snowflake. No matter what the size of your compilation units is, whether you call modules "modules", "classes" or "gorboodles", whether you allow odd features like mutually-recursive dependencies and build-time arbitrary code execution etc.: Your language fits into the same model as every other language. You don't have to NIH a package manager.
This last one is basically impossible at the current stage. Maybe somewhere down the line, if we manage to establish such a model successfully in a handful of languages and people see for themselves, but for now we have to just hold out.