I don't see how the Python 2 to Python 3 transition has been a "fiasco". It may not have been the fastest transition, but that really doesn't matter in practice.

Individuals and organizations with extensive Python 2.x software systems have been able to continue using those with no problems.

Development has been able to continue on Python 3.x without needing to worry about backward compatibility with Python 2.x. It is a cleaner language, in many ways.

Over time, we have indeed seen libraries, frameworks and users migrate from 2.x to 3.x as it benefits them. It's not something that's forced on them, causing disruption and anger.

And so we've gradually seen Python 3.x starting to become more and more adopted, while Python 2.x slowly fades away. It's a non-disruptive, steady transition.

If you want to talk about a real fiasco, Perl 6 is a much better example. Having no truly usable implementation after nearly 15 years, and thus basically no adoption, does qualify as a disaster in every sense.