Share your insights or perspective on the financial matter presented in the input.

At what point do index funds become unreliable?
As more actively managed funds are driven out of the market, the pricing of individual stocks should become less rational. I.e. more stocks will become underpriced relative to their peers. As stock prices become less rational, the reward for active investing will increase, since it will become easier to "pick a winner". Eventually, the market will reach a new equilibrium where only active investors who are good enough to turn a profit will remain. Even then, passive investment will still do roughly as well as "the market" since it has low overhead and minimal investment lag. There is no reason to expect the system to collapse, since it is characterized primarily by negative feedback loops rather than positive feedback.  The last few decades have seen a shift from active to passive investment because increased market transparency and efficiency have reduced the labor required to keep pricing rational. Basically, as people have gotten better at predicting stock performance, less active investment has been required to keep prices rational.