> a global coordination problem (everybody needing to have or install an exact version).

This coordination problem is already solved by PyPI. That's where Gentoo looks to find the build backend--just like everyone else that builds packages. The project owner had to be able to find whatever version they used there or they couldn't even build their project locally. So they should be able to depend on the fact that anyone else trying to build their project can also find that same version at the same place.

> getting every open source maintainer to upgrade their build backend version (whether uv-build, or flit, or anything else) every Tuesday.

You're missing my point. If I know that my project builds properly with build backend X version Y, and I pin that version in my pyproject.toml, I don't care how many newer versions you want to release. Unless I make a change to my project that merits updating my build backend to a newer version, I'm going to simply leave my project alone. I already know it will build fine with the version I have. Why should I push an update every Tuesday just because you decided to release a new version of the build backend, which I don't need anyway?

gentoo does not appear to download uv from PyPi https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-python/uv...