<p>I completely agree with you that no one has hit their hard cap, or what they are capable of learning. However, the hard cap is a significant factor on what a person is likely to end up learning. You could think of this as a graph x^(1/2), the x axis being effort put into education and the y axis being the result of that effort. This isn’t really a great example because this graph has no asymptote, but the general shape should work. People can also put in more effort, but that effort is realistically only going to do so much. So assuming people are remotely close in their effort this is something that is significant.</p>