>A key question arises: why are so few repositories type-correct?
The authors don't seem to ever discuss the fact that mypy version changes frequently make previously-passing code fail type checks.
I don't think I have ever upgraded mypy and not found new errors. Usually they're correct, sometimes incorrect, but it's a fact of being a mypy user. Between mypy itself and typeshed changes, most mypy upgrades are going to involve corresponding code changes. The larger your code base and the more complicated your types are, the worse it'll be, but it's basically an ever-present issue for any program interesting enough to really benefit from a type checker.
How many of those repositories were "type-correct" but only on particular versions of mypy? I bet it's a lot!
I don't have this experience. Can you give an example of a previously-valid codebase that failed typechecking unexpectedly on a recent MyPy update, that wasn't a result of a false negative bug in MyPy?
I obviously can't share it here, but my primary codebase at my job needs a few dozen changes every time we change mypy versions. There's a few places of false positives where I've had to comment `# shut up, mypy` and a `type: ignore`. Usually when I'm being clever with the subprocess module.