The more junior you are, the more likely it is that not only can your boss do your job, they can do it better than you can. The more senior you become, the less likely that is to be the case. How many CEOs could do all of the CFO, COO, CTO, and General Counsel's jobs? Zero. How many programming team leads can do the junior programmer's job? Basically/hopefully all of them.
But as you said, the more senior you become, things completely change. There isn't a CEO in the world that could do all of the jobs they hire other executives to do (finance, sales, tax/accounting, HR, marketing, technology, operations, legal). It's impossible.