Great initiative! Looks very promising @njs I really like that you’ve taken a holistic view of a larger scope of the problem, but not too large to make it impractical to solve.
Fwiw, I think it’s probably best to keep the discussion focussed on pybi and posy. The project is new, it’s explicitly stated in the OP that external pythons (such as a conda python) could be explored, but that it’s not a focus to get started. Let’s celebrate the project, not derail it into the same circular discussions we continue to have. conda is solving a different (bigger) problem than only python packaging. It’s really a better comparison to compare conda to nixpkgs (ie they are both “universal” package managers) than it is to compare it to any tool that only manages Python interpreter distributions and Python package distributions.