If you can build a piece of software and run unit and integration tests on it, it is more or less rot proof. That's because all those tests lets one delete or

If you can build a piece of software and run unit and integration tests on it, it is more or less rot proof. That's because all those tests lets one delete or replace components of the system without having to worry too much that the whole thing is going to break in some unknown way that won't be caught until it's in production. Without a good suite, the larger the system gets, the more people get scared of doing radical things to it because they are worried about breaking some critical functionality. As far as the not being able to build aspect goes, the main threat here is relying to much on closed source software and/or libraries that go out of support that no one can find the license, or critical documentation for, or worse manifests some critical unfixable bug.