To me this looks like better plan. Instead of unpredictable mechanism on when the messages disappear from you history, it's now predictable and you can act according to it.
I've used python typing heavily at work and was really useful to detect bugs and helped with developement.
> as evidenced by only 15% of repos passing mypy
That is no evidence. Of course, if you don't have mypy in CI (or pre-commit hooks or whatever), your repo will not pass mypy. But if you have it in CI, then you can rely on them being correct.
> tools like pytype or mypy will never capture all the complicated hacks possible
But they can either enforce everything being correctly typed (if you go to really strict settings), or yes, you will have some code untyped, so you will be careful around that part. It's not like all or nothing.