Python continued growing fast during the 2 to 3 transition, it was nowhere near dying. It was one reason it took so long, people were starting new Python 2 projects 5 years after Python 3 was released. Only around 2017/2018 the default for new projects kind of became Python 3.

Python 2 was used for such a long time because many modules only worked on 2, which was a direct result of the compatibility being broken. There was a catch-22 for a long time that nobody would upgrade to 3 because nobody else would.