In Python's case, as the article describes quite clearly, the issue is that the design of "working software" (particularly setup.py) was bad to the point of insane (in much the same way as the NPM characteristics that enabled the recent Shai Hulud supply chain attacks, but even worse). At some point, compatibility with insanity has got to go.
Helpfully, though, uv retains compatibility with newer (but still well-established) standards in the Python community that don't share this insanity!
I mean, you’re on the right track in that they did cut out other insanity. But unclear how much of the speed up is necessarily tied to breaking backward compat (are there a lot of “.egg” files in the wild?)
Not as far as I can tell, except perhaps in extended-support legacy environments (for example, ActiveState is still maintaining a Python 2.x distribution).
Helpfully, though, uv retains compatibility with newer (but still well-established) standards in the Python community that don't share this insanity!