You missed the point. The rigor makes things easier, not harder. It takes 10 times as many words to explain things the wrong way. And then you often need to know how to un-roll the wrong explanation into the right one.
How do you figure out a query’s computational complexity with the “set” explanation? You have to unroll it into the real thing (i.e., tuples) and then do a counting argument. If you start with the real thing, the counting argument takes less than a minute. If you start with the wrong explanation, you might never even reach a valid estimate.