I've thought this over, and my conclusion seems to be that people will take a reasonable amount of stress if they get a big performance improvement.

I absolutely agree that for some people, it will be too much, and frankly, I don't think that problem is solvable.

Well, it's solvable in the sense that breaking language compatibility is a choice that doesn't have to be made, I guess. The 2->3 transition was managed as effectively as such a big thing could be but it's for left the industry littered with a bunch of weird self-maintained python2 forks, compatibility layers for the C side of python. A 2x theoretical performance improvement doesn't pay down the engineering costs to make a switch if you recoup it in lower hardware costs later.

>hat still has a lot of Python 2 in house but has disallowed the creation of new Python code bases in favor of other languages.

That seems silly. It's not like other languages don't have their own major version migration pains.

What is the definition of a "big shop" here?

50k person years of work represented in the code bases and up, as an approximation. Big enough that to do the work you'll have to coordinate across multiple departments with different incentives for multiple years to get the transition made.