I don't think the parent is talking about Python 2 -> 3. I think they are pointing out that for many developers, moving to Python 3 happened very late. That means they ran 2.7 since 2010 (roughly 10 years now.) Now that they are on 3.x, they get issues with each new 3.x release.
The argument is that being on 2.7 for so long gave a false impression of the language being more stable than it is - developers just weren't using new versions.
Well to be fair the 2020 end of life date was announced almost a decade ago. That said, I know that fortune 500 companies that used 2.7 to develop new code in 2017 -- which was a mistake -- mostly because they were tied to old version of RHEL and they were too lazy to try to get Python 3 to work on an old version of RHEL.
So 2020 rolled around and the pypi libraries they were using were no longer supported even though RHEL would support python 2.7. So yeah, it was a really bad experience for everyone involved.
The argument is that being on 2.7 for so long gave a false impression of the language being more stable than it is - developers just weren't using new versions.