The main issue with the proposed changes is that it redefines what “the Python standard library” is.  Right now, users can mostly expect modules listed in the official Python docs to be available in their installation, regardless of how they got their Python.

I say “mostly” because a distributor may exclude tests from a binay package, split some extensions modules like _tkinter and _lzma in other packages, etc.  A big source of pain in the past was distributors splitting distutils, but I think they all stopped.

(A related issue is that some distributors backport many bug fixes and sometimes features, which is a pain when you think you’ve run your tests with “Python X.Y.Z” and it was actually “X.Y.Z+some-patches”.)