But I think a key point is that active investing already exists and dominates the market -- it's still like 85% active. People choosing to invest in passively managed funds are doing so because they see a higher return. Maybe the choice is sticky and people won't switch back if the returns begin to fall, but people still have to switch in.

So building up a surplus of passive investors requires people to continue to switch to a passive strategy after the returns between passive and active level out and reverse. And not just a little -- profits from investing are returned to investors, and reinvested. So if the equilibrium between active and passive investing is X% of the market passive, and we get up to passive investors at X+10% of the market, then the X+10% of passive investors will see a lower rate of return than the 90-X% of active investors. Which means -- unless those active investors take their surplus profits and dump them into then-less-profitable passive funds, before long the active investors will have 91-X% of the market and the passive investors will have 9+X%. Absent continued switching, the feedback of profit re-investment will push the balance back to equilibrium.