When investors are younger, they invest surplus cash. When they are older, particularly after retirement, they liquidate investments to generate cash. Given the demographics in the U.S., the excess return for merely belonging to the S&P 500 has been roughly 4% per year over the past 15 years; index funds have received disproportionate large inflows relative to the market as a whole. Aside from that, in aggregate, active equity managers benchmark to something that approximates the S&P 500. Belonging to the S&P 500 ensures a continuing flow of capital. Or does it? What will happen near 2020, when aggregate investment behavior changes from saving to liquidation? Belonging to major indices may not have the same cachet as investors liquidate their holdings to fund present needs. What was 4% positive in the 1990s could become 4% negative in the 2020s, absent a continuing move toward passive investing. I don't have a firm answer here, but I do have suspicions. I would be cautious of too much index exposure 15 years from now, to the extent it can be avoided. (And of course, this will be anticipated several years before the flows turn negative.)