Having been around through several recessions, my observation is that the idea that dumb index funds double your money every ten years is a theory that can [and has historically] run counter to the facts. One reason is that the dumb index funds are tied to indexes that simply swap out losing investments for winners when there's a loser in the index. It does not cost the index any money, but actual funds have to realize the loss of the loser and then pay for the winner.

Ten years ago the DJI was about 12k. Today it's 18k. That's not doubling every ten years. Over the long term upward trend it's a double since 1998...though if a person had had a pile of money to throw at the DJI in 2009, yes they could have doubled their money in seven years. In 2009, most people did not have a pile of money to throw at the Dow...or rather whatever pile they had was much smaller than it had been two years earlier.