I see this sentiment a lot, and it doesn't match my experience at all.
In my decade-old bubble of using Python professionally, I've never had an issue with virtualenvs. The few issues I might've had with dependency resolution must be so far in the past that I don't remember. But that's not strictly about virtualenvs. Likewise, pip could be clunky, but we don't have to deal with it anymore.
My niche is mostly backend. Other Python niches must be considerably worse in this regard.
you can? that's why go.sum exists. you can also use the replace directive for more advanced scenarios.
What Nix and build tools need to agree on is a specification or protocol for "building a software dependency tree". Like, I should be able to say 'builder = cargo' in a Nix derivation and Cargo should be able to pick up everything it needs from the build environment. Alas, there is simply far too much tied up in nixpkg's stdenv for this to be viable, so we have magic stdenv builder behavior via hooks when a build tool is included in nativeBuildInputs.
There's no real way to do that at a language level - we cannot have "Go has determined the package you are trying to fix has not met the versioning requirements proscribed so you cannot submit the patch to fix it"
What language dependencies do is what OSes would think of as "unofficial versioning" that is, an OS will let you install and run an unofficial version of some lib (we've all been there, right, multiple versions of some core library because one doesn't work with whatever you are trying to install), but they will not manage it at all.