> if rewriting chunks of code were enough in its own right, they would never have had so many critical security flaws.

Reducing defects is one of the main reasons (others being maintainability, readability, better integration, and similar) for refactoring and rewriting code. There's usually not enough time/money to do it, especially for large codebases.

I quite like rewriting parts of a codebase to modernize it, and I have often closed tons of bugs in a short time this way. It is definitely effective. But not as cost-effective as deprioritizing bugs into "won't fix" territory, which is what many companies like to do.