Firstly thanks for your work. As described in http s://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#fallback-behaviour the build-backend should fallback to setuptools.build_meta:__legacy__ but in http s://gitweb.gentoo.org/repo/gentoo.git/tree/eclass/distutils-r1.eclass#n1193 build-backend doesn't fallback to setuptools.build_meta:__legacy__ if pyproject.toml file is present. See also: http s://gitlab.com/mkdocs-i18n/mkdocs-i18n/-/merge_requests/16 (New accounts are forbidden from posting URLs in bug comments. Please wait at least 24 hours before posting URLs. We are sorry for the inconvenience.)
See also github.com/projg2/gpep517/issues/8
I think our current behavior is better. Packages that override build-system requires but not build-backend are extremely rare. I've seen only one, and there was no valid reason to use the legacy backend there. All things considered, the current behavior helps us find potentially buggy packages and changing it doesn't really fix anything that isn't broken already.
