You are precisely correct. Replacing Musk with someone else would lead to bigger changes than replacing the mailman.

But Why? It has nothing to do with Musk but everything to do with organizational form. He has an invention at his disposal called "Role of CEO". It is a magical invention that gives you the power to control what thousands of people do everyday. A mailman doesn't have this invention and obviously won't be able to decide what thousands of people do everyday.

There is nothing magical about Musk here. Since so few people get a chance at being CEO, it is hard to say if most people would be good or bad at it. And it is also hard to say how hard the work really is.