How was it a fiasco? The old version was supported for free for what, a decade? Usage of python is skyrocketing. I don’t know anyone who honestly prefer 2 over 3. The number of python packages released in one year now is probably higher than that of all years combined up to the release of python 3.
The process was not perfect, but in my book it’s a success.
It was a fiasco because it wasn't possible (until much later) to support both Python 2 and Python 3 in the same code base. This made for a gridlock between dependencies and users.
This is a misconception that has likely been compounded by the initial advice (by core Python folks, who eventually reverted their stance) on not doing this. For example, see this blog post https://aroberge.blogspot.com/2009/08/crunchy-10-released.ht... from 2009 for a non-trivial program that supported Python 2.4, 2.5, 2.6 and 3.1. It was a bit of a pain as it dealt with lots of unicode/strings, but it was doable. It became easier to support both Python 2 and Python 3 later on, especially once Python 3.2 was released.
The process was not perfect, but in my book it’s a success.