Well, in my ideal world maintainers would have put all possible effort to porting libraries to python 3 and put python 2 versions into legacy mode (e.g.: security updates, fork it if you want to continue on the 2-branch).

In my field what seemed to keep people on python 2 for a long time was numpy or scipy (or both, I do not remember which) which did not get a 3 upgrade for a long time.

Either that, or just call it something different, kind of like perl6. There is no perl6 distribution shipping a perl library or some perl.dll that clashes with perl5.