The interesting point is that, if enough people were investing in index funds (lets say, 3/4 of the market, for instance), it would be much easier for an active fund (or an active investor, or even a "smart beta" fund) to beat plain index funds, since companies would almost never be priced to their fair value.
In other words: it won't happen, because the very fact that index funds handle too much money for the market to be efficient would be arbitraged.