Exchange-traded funds have ushered in a rare type of revolution: a coup where an old order has crumbled without a drop of blood being spilled.

Investors have been the biggest winners. The low fees, transparency, daily liquidity and tax efficiency of ETFs were a blessing in the high-interest rates environment of the early 2000s when ETFs were first taking off. In today's environment of low yields and increased regulatory scrutiny they are doubly so.