There are way better metrics of function complexity, like how many branching points, how many loops, or even just how many levels of indentation.

TDD, OOP, Clean Code, etc are an attempt to solve very real problems. They are then applied as dogma to places where these problems are not evident. That's the issue. Of course these rules have their place, but always with a caveat and never applied over all possible places where they might fit. Very often, a better solution exists, as well.