The truth is, at least since the early 1960s category theory has been a much, much more powerful tool in many branches of modern mathematical research than set theory. (By the way, it can be an enlightening exercise, to try and restate the simple notions about sets purely in terms of maps - notions like 'B is a subset of A', 'C is a union of A and B', 'map f is injective/surjective', 'C is a Cartesian product of A and B', etc. Highly recommended!) Category theory is very deep and easily subsumes much, if not all, of set theory. The difference in power is staggering. Sets in general are now considered just one of many categories and play a subordinate role. I agree that set theory is more useful when studying math at an elementary level, but the sooner one starts getting used to categorical ways of reasoning, the better one's chances to make progress in understanding modern mathematics.