If most everyone puts there money in a index tracker it would go up or down based on contribution and withdraw rates versus actual results of the market.
So, while there is more contributing than withdrawing it continues to go up which would attract more money to it.
The risk is that there is so much in index funds that the valuations of companies will become distanced form what they're actually worth.
The likelihood of that happening given how much money there is to be made in finding the inefficiencies in the stock market are, in my opinion, very small.
* index funds are larger and larger share of investing
* Median Baby boomer about 63
* 1/2 Americans in stock market
* In 2017, over 62 million Americans receive SS benefits
* US Adult Population 247,773,709 = 123M in stock market
* Only 25 M generation X
* Millennials don't invest in stocks http://www.businessinsider.com/why-so-few-millennials-invest...
Conclusion Drain of stock market as sellers begin to out number buyers