IMO, it's an argument against diversification. The Nikkei's decade-long stagnation is proof that the indices that are widely considered "diverse" are not immune to stagnation. This is from a trader/investor's standpoint. Of course, the S&P can return 0% in a few year timeline, but the American economy would really have to stall to return 0% over a 10+ year time horizon. This might shock the Kevin O'Learys and Dave Ramseys of the world, but picking stocks that outperform in any macro environment is easy, you just have to have the stomach for vol and continuously invest.