The truth is we don't know at what point index funds will disturb the efficiency of the market. Malkiel makes a guess at 50% although I think he has an overtly

The truth is we don't know at what point index funds will disturb the efficiency of the market. Malkiel makes a guess at 50% although I think he has an overtly optimistic view of how much rational professionals drive the market. Regardless, let's suppose the mantra is "invest all your money in index funds until it breaks 50% after which bad things may happen". Do you think that if we manage to hit that 50% in the future, anyone will actually actively try to stop people from investing in index funds? Effectively there would be a lot of money very rich index fund managers who will be pushing to increase their fund sizes even further. So it's possible for extreme irrational exuberance to cause even index funds to harm the market.