"Over time, the code will be modified, and the integrity of the system – its structure according to that design – gradually fades. The code slowly sinks from engineering to hacking." (Preface p.xiv)
If the code works and doesn't ever need to change, it's perfectly fine to leave it alone. It would be nice to improve it, but unless someone needs to understand it, it isn't causing any real harm. (Chapter 1 p.5)