The Nikkei 225 is still down from 20 years ago. Its fate is a very plausible alternative history for the S&P.

Taking into account reinvested dividends, it's given an annualised return of 3.5% in the past 20 years. Source:

https://dqydj.com/nikkei-return-calculator-dividend-reinvest...

From that article:

"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 key is consistently.

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.

Based on numbers here: https://seekingalpha.com/article/2789035-s-and-p-500-index-r...

1940-1950: 9.3%/y

1950-1960: 20.0%/y

1960-1970: 7.7%/y

1970-1980: 5.9%/y

1980-1990: 17.2%/y

1990-2000: 18.2%/y

2000-2010: -1.0%/y

So only decade with negative growth.

Add in the depression and you'll probably see a bunch more...

You need to consider inflation. In 1970, the inflation rate was 7-8%/y on average, so the "growth" these numbers imply did not exist in reality.