[kenjackson]

I get that if your codebase is tangled enough, and your unit test suite is inadequate to "guarantee that your dataflow is more or less going to work" that maybe rewriting significant portions of it in a type-safe system makes sense.

Your unit tests can never guarantee anything about the dataflow of your system. That's not the purpose of unit tests. And unless you have a meta test system, that tests the properties of your unit tests, you can't get system-wide dataflow information.