This is one of those things that becomes clearly untrue if you try to apply any amount of rigor to the definition of "just fine".
I have yet to see a single major Python project claiming to use SemVer that didn't occasionally unintentionally violate the promise made by the versioning because you cannot always easily predict whether a change will be incompatible, and, even if you could, people still regularly just make mistakes because people are not perfect. Versioning is hard. Keeping promises about not breaking things is even harder.
That may be good enough for things that don't matter, but it's not good enough for things that matter a lot.
This is one of those things that becomes clearly untrue if you try to apply any amount of rigor to the definition of "just fine".
I have yet to see a single major Python project claiming to use SemVer that didn't occasionally unintentionally violate the promise made by the versioning because you cannot always easily predict whether a change will be incompatible, and, even if you could, people still regularly just make mistakes because people are not perfect. Versioning is hard. Keeping promises about not breaking things is even harder.
That may be good enough for things that don't matter, but it's not good enough for things that matter a lot.