Comment by manmal

Yeah, if done right. In my experience, such a reimplementation is often lossy, if tests don’t enforce presence of all features and nonfunctional requirements. Maybe the primary value of the early versions is building up the test system, allowing an ideal implementation with that in place.

Or put this way: We’re brute forcing (nicer term: evolutionizing) the codebase to have a better structure. Evolutionary pressure (tests) needs to exist, so things move in a better direction.