And an enterprising code keener will insist on rewriting that legacy codebase to eliminate all "smells", replacing battle-tested, bug-free-but-bad-smelling code with fresh, elegant, untested and nice code.
Every generation relearns this the hard way, this blog post is 23 years old today and still 100% on the money https://www.joelonsoftware.com/2000/04/06/things-you-should-...
I'd like to point out that a mature/legacy codebase contains every example listed under the "smell" section of the article, sometimes even within the same file. This creates a great deal of complexity to unwind beyond the mere coding of it all.