The transition was slow (Python 2 was supported in parallel to 3 for a long period of time so there was more than enough time to migrate), relatively easy to do, and brought huge benefits to the ecosystem.
It could have been done and forgotten in 2 years if some community members had not been dicks about it.
It really seemed that the issue was mostly "Unicode is really hard if you haven't thought _deeply_ about it" because application code, library and framework code, _and_ "system" Python code had to go through several iterations of "here's how to do it right" before something reasonable came out that solved issues for most / all stakeholders. And the Swift / Raku people are still sitting on the sidelines smiling, knowing that there are a lot more "lenses" to view text through that Python doesn't make easy (but neither does Haskell, JS, or Java, so there's good company in good-enough-for-now land).
JS devs have been transpiling for years now and it works.
Python should have released a conversion tool that could automatically convert most stuff and identify the stuff it couldn't convert. This kind of directed upgrade would have made the process much easier for developers to actually accomplish.
The transition was slow (Python 2 was supported in parallel to 3 for a long period of time so there was more than enough time to migrate), relatively easy to do, and brought huge benefits to the ecosystem.
It could have been done and forgotten in 2 years if some community members had not been dicks about it.