I don't get why people expect no breaking changes

> With hindsight, it probably would have been better to take distutils from Python 2.7 or something like that and brand that as the implicit default build tool.

distutils specifically would just not have cut it. By the time it was removed from the standard library it was considered an unmaintainable mess that nobody really knew what to do with.

It would have been nice for Setuptools to be able to extract a "core" build system that is restricted to the part that actually implements PEP 517 (discussed in other comment threads), which could then be used separately from the part that enables workflows on the developer's machine. Flit and Hatch work this way (using flit-core and Hatchling respectively). However, a "Setuptools-core" would have to have been restricted to only handling projects that can configure everything in `setup.cfg` (and `pyproject.toml`) since `setup.py` is the same file used for implementing, for example, arbitrary new sub-commands for command-line developer use, or for setting up tests in arbitrary Python code (subsequently run with `setup.py test`).