Well yes, sure, of course.

And like you said, maybe Python isn't the right language in the first place for mission-critical life-is-on-the-line software.

But if you have already gotten yourself into a position where some piece of your business infrastructure is dependent on an obscure bit of hard-to-port-to-Python-3-and-maintain-exact-behaviour Python code, then it is exactly the "2to3 transition that's shaking up your house of cards", no?

And, furthermore, like you said, if you find yourself in this position, you should be looking at some other language entirely rather than porting to Py3, eh?