You're assuming a lot. What about codebases with python2 third party dependencies that don't work in python3? Now you have to port that entire library as well,

You're assuming a lot. What about codebases with python2 third party dependencies that don't work in python3? Now you have to port that entire library as well, or write it yourself while crossing your fingers that it is well documented and easy to work through. What about codebases without decent test suites? I'd argue most production codebases don't have good test suites, or at least the most complex of code is usually poorly tested. You'll end up spending most of your time digging for regressions especially if your code creates large amounts of user interfaces. What about code bases that were written by scientists, mathematicians, or other professionals who may not be as fluent in writing "good" code?