I agree reproducibility is an important problem, but the author assumes that dropping backwards compatibility is often necessary at _some_ point in order for a

I agree reproducibility is an important problem, but the author assumes that dropping backwards compatibility is often necessary at _some_ point in order for a language to improve, and that the improvement is at _some_ point worth it. I think in this context that's the answer to your "Why?". There are all sorts of issues with reproducibility of scientific work beyond py3k vs 2.x (virtualenv is one way to address that one) like all your packages, for instance, and all the libraries they rely on, which are necessarily going to change at some point. That's part of a bigger discussion. Hoping your work from 5 years ago is going to run just because you never upgraded to py3k is probably not going to solve the problem.