"But the point at which passive investing becomes self-defeating is a bit like the point at which the gradient of the Laffer curve turns negative, and tax hikes cause revenue losses rather than revenue gains: both points are far beyond any state of the world that obtains in real-life America. Passive investors are still a minority of all stock-market investors — and, what’s more, they could easily become a majority without doing any harm to the markets’ price-discovery abilities. The only thing that matters is that there’s a reasonably large number of active marginal price-setters. Since there always will be a reasonably large number of active marginal price-setters, no one ever need fear that the rise of passive investing is going to become self-defeating."
and highlights the gains to be made from cheap and systematic investment.