I get this, though: porting code affects all parts of the code base, so it generates git conflicts all the time. It's way more of a pain to do than routine feature development or refactors. It gets difficult to have part of the team work on features while another group works on the port to a new version, because that generates a ton of conflicts. It's easier to get the entire team working on the port.

I had to manage a mature codebase upgrading Ruby versions, and it was painful. And that is fairly mild by comparison; not the big jump from Python 2 to 3.