By experience, people with large Python projects often overblown the difficulty of porting in their head. Unless you have a very rare irreplaceable dependency or some terrible C extension, porting is easy.
It's tedious yes. Boring even. But most projects get away with 2 weeks of investment. And yes, it pays back. Python 3 is a vastly superior language when it's about introducing less bugs or debugging existing ones. It's just not stuff what had be sold to the public. People want to hear about better perf or fancy features, not better error messages or banned logical errors.
The hardest code base I saw ported was the Twisted project. It's a good counter example: it was long, hard, and required very gifted people (thanks Hawkowl !).
But most projects are not like that. They are full of dumb operational logic that can be ported 80% by 2to3, and a bit of manual fix.
The fact is, people love to complain, so you will hear massively people with that particular example from the movie industry, or this guy who coded this Fortran extension that was in such a tough situation. Well guess what, that's not what most migrations are about.
Most migrations are Django/flask websites, math teachers exercises, physicists scripts, sysadmin tools, etc. Straightforward stuff. I should know, I've been moving from industry to industry for 10 years, all my clients are different, but most of them end up with similar stuff in their repo because Pareto is a thing.
Nonetheless, the screams from the first ones scared the later.