Macro Investor deserves credit in pointing out that leveraged ETF UPRO would have performed very well over that last six decades. But I just want to point out that leveraged ETFs, in particular UPRO, is built to be actively managed. It makes little sense to buy and hold.
My argument can be seen in my comment on his article, repeated below.
Your argument indeed holds for the past six decades or so. But nobody should take away complacently that S&P 500 is not volatile and as such one can buy and hold UPRO with great peace.
(1) S&P 500: 1527 on Mach 24, 2000, and 1531 on Feb. 19, 2013. So it finally got back to its original value after almost 13 years. If you have bought and held the imaginary UPRO, however, you would have lost 83% of your investment after the nearly 13 years!
(2) This is the far-worse part. S&P 500: 1565 on Oct. 9, 2007, and 677 on Mar. 9, 2009. This is the recent financial crisis. S&P lost 57% of value in only 17 months. However, UPRO would have lost 96% of its value. You basically got wiped out during the 17 months period.
So, regardless of how the world evolves over the long term you would still be much better off actively managing your UPRO position than just buying and holding it. You certainly should get out when market looks frothy; so you can get back in when it is attractively-valued and fear is running rampant.
Furthermore, as a prudent investor one should be prepared for the possibility that the world has changed a lot due to a multitude of factors. So, the next six decades might turn out to be rather more volatile than the past six. In that case, you might be wiped out holding UPRO or any other leveraged ETF for the long run.