> It's interesting how Nix's selling point is that its non-standard file system structure allows multiple versions/flavors of the same package to be installed a

> It's interesting how Nix's selling point is that its non-standard file system structure allows multiple versions/flavors of the same package to be installed at the same time, yet if I understand correctly, it often has one version per package per Nix version? You're talking about Nixpkgs, and it's a cultural choice. In the past there used to be more of this, where e.g. the Haskell community had lots of variants of the same library all at once, because users and other libraries wanted them. But it became untenable to maintain and you in practice have to do tons of effort to vendor things. And once many of the subcommunities began moving to auto-generated packages (e.g. generating Nix packages from Haskell packages), it becomes much easier to have a single globally consistent repository of versions that all are known to work together, generated by the host-language dependency resolver, with minor fixes applied on demand. There are some things that do have multiple versions in the main repository, e.g. LLVM. But in practice having like 10 versions of everything is a huge chore and only done on demand in practice if users want it, and it has real overhead cognitively and in build time and in maintainer time. You can define your own Nix flake with every version of every Python library in it if you want. But in practice people sort of pick versions of things that make QA a real chore. EDIT: Also, for like 99% of cases, what users think is cool but hardly do is to run both pieces of software at the exact same time concurrently. They typically just want atomic upgrades and rollbacks. For example, you change your git revision at work and get whole new tools, your old tools still work if you go back to the old git revision at no loss. So the need to only have one version "globally" under one name isn't a big deal in practice, because people often are only dealing with one global name. They just don't want their packages to get corrupted during an upgrade or switchover or something otherwise breaks.