I suspect that musicians are earning less than they did a decade or two ago. There's no way for a Michael Jackson to sell a Thriller album in today's fragmented, digital marketplace.
It doesn't have to be this way. Germany requires that the board includes employees. Japan has also succeeded, perhaps by applying social pressure, in keeping executive salaries reasonable in most cases.
Unless you want to claim that Lady Gaga's performance is superior to that of Johnny Cash, I don't think you can explain the increase in musician pay by performance increases.
I'm all for paying people whatever it takes, even if that means some people make previously insane sums of money. But they should have to make their company MORE profitable, not less.