>One way to see that is to consider saving a reasonable amount (say 10%/year after-taxes) of an upper middle class salary by putting it into the stock market every year of employment for ~45 years, and simply assume that over that time the stock market exhibits something close to a 10% CAGR.
Do the math. You'll see that will generate a lot of millionaires - and multimillionaires.* What's not to sustain?<
Just so that I am clearer on your example, can you define the "upper middle class" salary that you're using in this example? A range would be very useful!
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