I think it's a fair point to say "the move from 2 -> 3 caused an enormous burden for library maintainers, and many packages aren't going to be upgraded for 3".

I think it's a fair point to say "the move from 2 -> 3 caused an enormous burden for library maintainers, and many packages aren't going to be upgraded for 3". There's no question that this is true, inside the scientific computing community and out. It was arguably worth it to get a better language, but still, the transition doesn't come without cost. However, that's an entirely different point from "the move from 2 -> 3 means that there's a bunch of science that isn't reproducible anymore". No there isn't, just use older versions of Python. Python 2.7 isn't going to self-destruct in 2020, you can still install it if you want to verify and examine an old result.