Yes, that's what I'm suggesting. I know a big python2 codebase (SageMath) that has been "close" to python3 adaption for several times within the last 5 years but had to call it off every time because something came in the way. By the time the next serious attempt was started, lots of the changes had rotten. Anything that could be done incrementally (python 2.7.5, from-future imports, prints with parentheses) has been done long ago.