Why Calculus?

Sometimes people ask me, why should students study Calculus? Today I saw this figure in a major magazine. Now, the law of diminishing returns means that there is a place where the *slope* (derivative) of the function starts decreasing, it is not the place where the function itself starts decreasing. What they marked is a local maximum. For this function, diminishing returns starts at technology use 0. Why study Calculus? Because the difference between "what I have" and "t