So switching Python 2 -> Python 3 is a higher cost than Python 2 -> Go?!?

Yes if you add in amortized benefit of extra performance and static type checking during compilation.

benefit(2->3 + performance benefit + maintainability) <

benefit(2->Go + performance benefit + maintainability)

The time cost of 2->3 will be less but if I know I will get better memory footprint, and faster response times I might be willing to invest some more time into it.

Or another way to look at it, once one sits down and rolls their sleeves looking to re-factor/renew/refresh the code base there is a good chance they will look around them and see what else is out there.

Or something else, yes. If the cost is potentially going to be big enough to convert for a large code base (keeping in mind testing might be a huge part of it - having to potentially re-test everything), and for very little benefit (well, unicode possibly, but it's not like Python 3's really any faster, and the GIL's still there), it makes you evaluate if it's worth enduring a bit more transition pain to convert to something else completely which would bring the possibility of new features / better speed.

It's also quite an issue for COTS software - sometimes to jump forwards and clean up APIs they have to do a huge refactor, sometime even a re-write of the API. If this is troublesome enough, it's worth taking the time to look into the competition.

I wouldn't consider rewriting in a complete different language, without any support for idiomatic Python code a "bit more transition pain".

GIL only exists on CPython, there are other implementations.

GIL also exists on PyPy.