it's not really about how the software works. the source code says what it's doing. the thing is the source code is the product of a theory; a theory determines

it's not really about how the software works. the source code says what it's doing. the thing is the source code is the product of a theory; a theory determines how you respond to the world. if that theory is lost, then updating--in the long-term sense--becomes hard, because the theory that you form might (probably) be incompatible with the theory that made the software. this incompatibility might make producing functioning software difficult, but more likely, the mismatch will make choices that don't make sense to, say, users, appear to be the obvious thing to do.