[Johnny555]

That might be workable idea if the guy writing the replacement code has such a perfect understanding of the original code that he can spell out every assumption and edge condition that the code covers (as well as write tests to cover all conditions). The best you can hope for is comments and tests that cover the new author's impression of what the old code was doing, which is not necessarily the same as what the old code did.

In this case, since he apparently doesn't feel that he has that perfect understanding, he left the original code there, so when someone says "Hey! I counted on behavior XXX, this change broke my workflow!".

And any change no matter how trivial or "correct" always breaks someone's workflow.

https://xkcd.com/1172/