This is a very important story that has been under-reported/recognized. Jack Bogle of the $4.2 trillion Vanguard funds has said this for years (probably decades?). Great NYT (4/14) article on Vanguard https://goo.gl/ayCfoV . Vanguard is the king of index funds taking in $1–2 billion a day and is effectively owned by the fund holders.
One consideration — as these indices outperform and more money is simply dumped in indices, the underlying stocks might be a bit inflated (although I would not bet against that). And naturally, if the market declines, fund withdraws will put even more pressure on the stocks in the big indices…feeding more withdraws.