I disagree. Incremental changes are not always capable of reaching point B from point A. A large, extremely messy code-base can make even trivial changes unsaf

I disagree. Incremental changes are not always capable of reaching point B from point A. A large, extremely messy code-base can make even trivial changes unsafe, let alone large changes. The effort required not only to figure out what point B is, but also how to safely reach it from point A is immense. Much much harder than a rewrite. However, it is a very good idea to meticulously read the bad old code and write down a list of things it handles -- to make sure none of it is missed in the rewrite.