The 'rule of 72' is a quick and easy rule of thumb to determine how long an investment doubles, and improve how you think about compound interest. For example it takes approx. 10 years to double an investment at 7%, or 7 years at 10%.
It's currently up 24% YTD.
TBH I don't think looking at the last hundred years is particularly valuable. Nobody invests over a hundred year timespan. Most importantly, look at a graph of interest rates over the past 40 years. There is a fundamental "new normal" of extremely low rates (or, rather, negative rates for much of the world), making it extremely difficult to earn that 7% without taking on a very large amount of risk.
It may be true only for the people that have invested ALL their money as a lump sum right before the index crashed (at the worst possible time).
For anybody else, those that say invested in the previous few years into a passive index fund, and continued to invest in the next few years they would have broken even within two years then probably quadrupled their money by today.
Keeping the same amount in a bank would have turned it into 30% less - a decrease similar to what the "crash" would have caused.
People need to be taught not to be afraid of investing. I know many smart folks, some who are engineers, who were scared of investing until they were in their mid 30's. My dad taught me about investing when I was a teenager. You do this right, you can retire in your 40's or 50's, never have to work again if you don't want to.
You might luck out, maybe hit it rich on startup stock options by joining the next FAANG company. This is unlikely to happen. Investing in the stock market, week after week, year after year, decade after decade... It's almost guaranteed.
One of my biggest regrets, coming from a family/socioeconomic group where nobody invested, is not beginning to invest as soon as I had enough disposable income to safely do so (in my mid-20s) instead of in my mid-30s.
If you're 33 and working in IT, you should be holding stocks and hoarding that fantastic growth (and slowly, over time, scraping the gains off into bonds).
You bank savings are only guaranteed by FDIC to $300k. Chances are they are not giving you an interest rate that will keep up with inflation, either. At best it's not gaining value, and is in all likelihood losing value by tiny amounts as inflation eats away at it.
Keep in mind that costs for elderly care have gone nowhere but sky-high in the past decade or so. It's not crazy to plan on paying 10-25,000/month once you reach your 80s or 90s (or more if inflation rises).
Only these losses are permanent :-/ whereas the stock market recovered and quadrupled...
"What is the average annual return for the S&P 500? ... roughly 8%"
Of course, past performance is not necessarily indicative of future performance. But as a mental arithmetic trick, the rule of 72 generally works.