For what it’s worth, Perl 6 can load Perl 5 libraries: https://github.com/niner/Inline-Perl5
Python3 broke compatibility for spurious reasons. The language didn't change that much, it could have kept compatibility with minor concessions.
> Java or COBOL maybe have done a better job of not breaking backwards compatibility...
Literally every single other major language has done a better job at it. That's a big part of why they are major languages. Also, most of these are statically typed, so any minor breaking change is far less risky to pull off.
> ...but Python 3 isn't exactly Perl 6 here.
Well, maybe if Perl6 didn't happen Perl wouldn't be irrelevant today.
C++'s upgrade woes make Python 3 look downright trivial in comparison.
If you're talking about ABI breaking, that's a different story. Python breaks the ABI in every major release as well. That generally requires a recompile, not a major code migration.
But hey, if some 3rd party group effectively forks the language and maintains a Python2 branch after the official EOL date, good on them. But I'd argue that using that is risky in the same way that using a Pale Moon or Waterfox is (arguably) riskier than using mainstream Firefox.