The reason you can "download a package and its dependencies" with other package management systems, is not because they are technically superior, but because they've had countless man-hours of contributors making sure that their central repository contains packages which work well together.
As soon as any assumption changes, like for example, adding a third-party application repository, then you need to concern yourself with all of the issues mentioned above, or you'll end up wasting your days on manually building and installing packages locally.
The point of Nix is to do this once only. If you build and it runs, it should do the same for everyone, regardless of what other repositories they use. Package management becomes completely modular and does not depend on some centralized state like a central software repository, or the state of your machine at time of install.
Nix isn't trying to do too many things. It's actually much, much simpler than using a bunch of semi-compatible tools which all attempt to do similar things in ad-hoc ways. Nix is just a dependency management system where dependencies are specified by their content hash. Your machine contains a content-addressable store of objects which can be precisely referenced by any other object. It takes the guesswork out of dependency management. Other dependency managers effectively run a search engine to try and find an appropriate dependency from some search terms.
As soon as any assumption changes, like for example, adding a third-party application repository, then you need to concern yourself with all of the issues mentioned above, or you'll end up wasting your days on manually building and installing packages locally.
The point of Nix is to do this once only. If you build and it runs, it should do the same for everyone, regardless of what other repositories they use. Package management becomes completely modular and does not depend on some centralized state like a central software repository, or the state of your machine at time of install.
Nix isn't trying to do too many things. It's actually much, much simpler than using a bunch of semi-compatible tools which all attempt to do similar things in ad-hoc ways. Nix is just a dependency management system where dependencies are specified by their content hash. Your machine contains a content-addressable store of objects which can be precisely referenced by any other object. It takes the guesswork out of dependency management. Other dependency managers effectively run a search engine to try and find an appropriate dependency from some search terms.