Python 3 created a huge amount of unnecessary effort and made a lot of people hesitant about using python again
Python 3 is a hugely successful language and implementation, and almost no one regrets that it exists aside from a few noisy holdouts, and people who never liked any Python anywhere at any time.
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...
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.)