Earlier quoted context omitted.
That wouldn't be large enough a change to warrant a new major SemVer. I'm talking about things like python2 -> python3.
It's clear the py2 to py3 migration was painful but I'm curious to hear how you would apply "they should fork / rename the language" here. To me it just feels like semantics. They said "here's a new major version of python" when they could also have said "we have forked python 2 and we're calling it python 3. We think it's better and we will probably abandon python 2 at some point".
But it's about setting expectations.
The only problem I ever had with py2 to py3 migration was that it was even possible to have the same codebase run against both, when languages are incompatible to such a degree (most notably, basic type has changed). It basically forced people to make the worst use of the Python dynamic nature (as soon as the stdlib started doing that, there was no going back).