Python2-3 is a worst case example where the benefits were small or negative compared to cost. There are also plenty of very successful languages used for large

Python2-3 is a worst case example where the benefits were small or negative compared to cost. There are also plenty of very successful languages used for large complex problems and have broken compatibility in various ways over the years. You are right nobody wants to pay to rewrite it but if the tooling takes care of it for you nobody cares about it. Python 2-3 had tooling that, for larger customers and projects, often could not do even 50% of the conversion, and when it came out the other side, nobody felt it was really better.