Whether or not a software project succeeds is substantially influenced by politics.
When I read your post a strong anti python 3 feeling is conveyed.
It may well be that it’s impossible to upgrade such a company/code base when the engineers have such a negative attitude.
The barrier was perhaps not technical but about the attitude of the people.
There were lots of python 2 developers who were rabidly anti python 3. Some of the most well known python developers were aggressively very publicly anti python 3. Imagine a python 3 upgrade project with that attitude prevailing. No chance of success.
Today python 3 is more popular than ever before and is arguably the most popular language in the world after javascript. Lots of people love python 3.