Unique and Difficult to Replace

In comments to my post about athletes and biologists, reader Rob notes:

ARod makes that amount of money because entertainment makes tons of money. And he isn’t easily replaced. You’d have a better argument as to why are money mangers, who are fairly easily replaced, worth so much money.

Your thoughts.

(I think Marginal Revolution may have covered something along these lines not long ago. But I’m running late and I don’t have time to check. Apologies.)