"While 10% might be the average, the returns in any given year are far from average. In fact, between 1926 and 2014, returns were in that “average” band of 8% to 12% only six times."

So if you are investing on borrowed money with a guaranteed payout of 7% a year, that is not good for you.

The S&P 500 returns are wildly variant, and the 9~ % only comes from averaging a century of growth. Whole decades might see a loss.