The biggest problem of the Python 3 release was that it broke a bunch of _Python_ code. That's pretty different from changing the C API. But sure, it has ups and it has downs. One of the downs is that Python, despite a lot of announcements over the years, still is struggling to become significantly faster.
yeah. Python is somewhat trapped here. because Python is slow, real work gets moved to C, and the C api makes it almost impossible to speed Python up. If Python had made the API changes needed for speed 20 years ago, there would be way less C code, so further changes would be easier, but that ship has now basically sailed
