I think you're really under-rating how important it is to be able to do something like "pip install 'requests==1.0.5'" or whatever, in order to reconstruct the past state of a project. If requests hasn't pinned its dependencies, that command will simply not work. The only way you'll be able to install that version of requests is to manually go back and piece together the whole dependency snapshot at that point in time.

There's pretty much no point in setuptools automatically installing library dependencies for you if you expect the library dependencies to be unpinned. In fact it would be actively harmful --- it just leads people to rely on a workflow that works today but will break tomorrow.

You're asking for an ecosystem where there's no easy way to go back and install a particular version of a particular library. That's not better than having version conflicts.

The other thing I'd note is that it's quite an understatement to say that pinning dependencies makes life "slightly easier" for library developers. We're not going to accept builds just breaking overnight, and libraries that depend on us aren't going to accept us breaking their builds either.