I have been responsible for an exception to this rule. We were new people coming in to take over, decided to go for rewrite, and were successful.

But part of the reason for the rewrite was that the logic (translation of business domain knowledge into code) in the old version was wrong resulting in unstable behavior. It could have been fixed in the old version (very many places) but the lack of a test suite too tipped the scales for rewrite (with test suite).

You can say that we learned the domain (at least) as well as the authors of original system after coming in, but won't that be the case of any successful rewrite?..