I don't disagree that python3 is hugely successful, but that doesn't mean the Python 2->3 pain was necessary. Certainly many Python users started using it too recently to remember it though.

I saw a solid dozen Python 2 projects leave Python entirely across a couple companies back during that timeframe.

The Python ecosystem has been growing overall especially because of the success of things like Pandas, but a lot of backend/fullstack web app programming did move away from it and never looked back.

(Though you might say the more interesting question is: would they have moved away to things like Node for async/perf or JVM-stuff for "maintainability of old large codebase with lots of devs" issues? Maybe? But at this point Python has added in a lot of things from those languages; but maybe if they'd been there five years earlier with a cleaner upgrade story the migrations wouldn't have happened.)

python3 had no compatibility mode, so everyone needed 100% of their dependencies to migrate. This was so painful that some teams abandoned their legacy python2 code and reimplemented in languages with better back compat stories.

Python 3 was announced way ahead and came with migration tools that worked pretty well. Besides character handling most of the stuff was pretty similar at the beginning and I never understood why apparently nobody or only a few projects made the switch early on.

That lead to a chicken and egg situation: if you depended on those libraries that did not migrate to python3 you where stuck at python 2 as well.

I believe being nice to the community and supporting python 2 for a long time was a mistake. They should have made a hard break and enforce the migration...