By a search, Gentoo accounts for less than 1% of all developers. If uv-build is more conformant to people, probably volunteers (and if they're using it, it probably is), I don't think the Gentoo people have any right to want them not to use it because it makes their lives harder. They can request, and they do request nicely, but they also present the drawbacks for them as if they are absolute, and even claim that "its benefits don’t outweigh the drawbacks".

You think Gentoo isn't important, they think the build backend doesn't matter. They're asking. Nobody's compelled to listen.

This is their request

I'm sure it accounts for a much larger proportion of people using exotic or legacy systems. In any case, I think most people are using uv-build either because it's a default for uv now or just because it's hip and new. If a project absolutely needed it then so be it, but if you knew it was going to make life difficult even just for one person and no benefit to you then you'd probably choose something else.