I agree. So essentially the "rust" in a codebase is the differential between what your developers know about the code and what knowledge is embedded in the cod

I agree. So essentially the "rust" in a codebase is the differential between what your developers know about the code and what knowledge is embedded in the code. Since it is much harder to read code than write it (at least, for "awful" codebases), once code has rusted enough it is best to just throw it away. On the flip side, if you are able to keep your developers up to date on the codebase, you will save yourself a lot of trouble.