Or it doesn't. Because "software as an organic thing" like all analogies is an analogy, not truth. Systems can sit there and run happily for a decade performing the needed function in exactly the way that is needed with no 'rot'. And then maybe the environment changes and you decide to replace it with something new because you decide the time is right. Doesn't always happen. Maybe not even the majority of the time. But in my experience running high-uptime systems over multiple decades it happens. Not having somebody outside forcing you to change because it suits their philosophy or profit strategy is preferrable.
Or more likely the 'whole' accesses the stable bit through some interface. The stable bit can happily keep doing it's job via the interface and the whole can change however it likes knowing that for that particular tasks (which hasn't changed) it can just call the interface.
