Why exactly is having this insetup_requires insufficient? Or is that only going to work withpyproject.toml?

I don't know; this came in with the original PR#18971; would have to ask@anntzer for the exact reason.

See#18971 (comment) and#18971 (comment): the two options were either
(a) don't have a runtime dep on setuptools_scm, and accept that inplace installs get the wrong version if one doespip install -e . and then update the git repo (the version comes from_version.py and is only updated when setup.py is run, either directly (e.g. when recompiling) or via pip); or
(b) make setuptools_scm a dependency so that the version is always correct.

I was actually in favor of (a) (as described in that thread), but didn't feel that strongly about it. I guess that's another opportunity to go back to (a) :-)

accept that inplace installs get the wrong version if one does pip install -e .

Do you mean that the wrong version gets reported, or that the install doesn't work? I think reporting the current commit in the version is nice, but hardly crucial.

The wrong version gets reported if one doespip install -e and then updates the git repo (and even then, there are probably workarounds possible e.g. using git hooks...).

As noted in#21783, my solution has been to import and use setuptools_scm, then fallback toimportlib_metadata. Given the fallback, it seemed fine to treatsetuptools_scm as an optional dependency and only list it as a "build" dependency. It's not perfect, but I've gone to statically listing all package metadata insetup.cfg.