In order for a part to be entirely interchangible, you should choose the smallest common denominator of all parts to rely on as the highest value that you can get from one. I think that one must find an equilibrium but it requires active management and careful thinking to be executed properly.
Or to have great onboarding, people learn a lot, are effective quickly, and leave after learning enough. The know-how is then embedded in the process not in people, which guarantees stability, while people are guaranteed to change.
If you can embed all of your organization's knowledge in a few weeks of a single person's study, then you don't have much knowledge to begin with. Such stuff might be good for a small factory, but for nothing more complex than that.
