While I can't say for sure, one or the reasons I seem to recall from the Python 3 transition was that the Python 2 design was pretty much a dead end. There where so many limitation and wrong design choices that it would keep the language from moving forward. That does seem a little aggressive, but it does feel like Python picked up a lot of steam once Python 3 was viable (something that happened way earlier than many care to admit).
Our code base wasn't huge at the time, a few 100.000 lines of code. Getting everything running was a week of work for three people. Sure many had way more complicated code, and some depended on some idiosyncrasies of Python 2 making thing more complex, but a lot of people acted like the Python developers shoot their dog. Mostly these people either simply didn't want to do the work or their particular niche was made ever so slightly more complex... or they depended on unmaintained libraries which is bad in it's own way. Python 3 was always going to be able to do everything Python 2 could, yet a large groups of people acted as if that wasn't the case.
Still not the best transition ever devised, we had to wait until 3.2 to get the speed to a point where it's wasn't an issue for all but the largest users.
The Python 3 upgrade process for many projects was incredibly painful. "Mercurial’s journey to and reflections on Python 3" should be required reading for anybody with rose-tinted glasses of the migration.
There was, of course, a Hacker News thread discussing the article, and a fair few people decided to blame the Mercurial developers for handling the migration inelegantly. Because that's how you win over an audience of developers - reassuring them that if Python has a backwards-compatibility break, Python fans will go out of your way to try and blame you for writing bad code. And not, perhaps, the fact that Python was missing things like a u string prefix and % bytestring formatting until 3.3 (2012) and 3.5 (2015!!!) respectively.
If I sound peeved, I really loved Python in the 2.x days, and the way the 3.x transition was handled broke my heart and prevented me from using the language for pretty much an entire decade. There are lessons to be learned from the transition, but not if we ignore the real problems that the transition caused. More importantly, we need to recognize that Python is not the Python we know today because of how "well" the transition was handled, but because Numpy and Matplotlib swooped in and gave Python new niches to excel in at just the right time.
All well and good when u have an active dev team who knows the code. Have fun walking into a code base that has just been running for the last 5 years and all the consultants that created it have left.
Our code base wasn't huge at the time, a few 100.000 lines of code. Getting everything running was a week of work for three people. Sure many had way more complicated code, and some depended on some idiosyncrasies of Python 2 making thing more complex, but a lot of people acted like the Python developers shoot their dog. Mostly these people either simply didn't want to do the work or their particular niche was made ever so slightly more complex... or they depended on unmaintained libraries which is bad in it's own way. Python 3 was always going to be able to do everything Python 2 could, yet a large groups of people acted as if that wasn't the case.
Still not the best transition ever devised, we had to wait until 3.2 to get the speed to a point where it's wasn't an issue for all but the largest users.