Unlike competitor T. Rowe Price Group, which emphasizes its hub of analysts over individual fund managers, Fidelity's reliance on star managers like Danoff poses a problem for it over the long-term, Rosenbluth added.
"There's a very small handful of managers who are known, and like everybody else they are getting older and sooner or later they will be retiring," Rosenbluth said. "At that point it's not inconceivable for investors to say I'm just going to choose a Vanguard 500 index fund from now on."