Earlier quoted context omitted.
Musicians are used instead of imaginary button pressers because this is a real world effect. Think about it this way...assume everyone is paid fairly for the value of their work, no "exploitation" (although that's a tough thing to impartially define). A factory worker who does 10% of the work in producing 1000 widgets per day can enjoy a much higher standard of living than his grandfather who made 5 widgets a day wit…
The modern doctor is far more productive. She's healing 10 highly productive units. In fact I think that could generalise for all service based work. Providing a service enables people to not have to do the thing themselves. As such the productivity of the service can be considered a factor of the productivity of the people they are providing the service for.
A BMW factory that produces 100 cars/day might be more valuable than a Toyota factory producing 100 cars/day, but it wouldn't be more productive on a per unit basis.