I don’t know why you are ignoring the concept of opportunity cost to make an argument.
Did you drive to work today? Did you learn everything about the tensile strength of nylon seatbelts before you buckled up? How about tarmacadem design before you exited your driveway? Or PID controls theory before you turned on the HVAC?
The point I’m making is that some people disagree about how much you need to know. Some people are ok with knowing just enough to get a job done because it makes them more productive overall. So the question still stands: How do you gauge when learning is enough? To my point above, I think it comes down to whether you can get the job done. Leaning beyond that may be admirable in your mind, but not exactly what you’re being paid for, and I think some experts would consider it a poor use of time/resources.
Do you care if your subordinate wrote a good report using a dictionary to “cheat” instead of memorizing a good vocabulary? Or that they referenced an industry standard for an equation instead of knowing it by heart? I know I don’t.