This is a provocative but deeply misleading calculation. It assumes that our no-college-S&P-investor is a stoic enough soul to live on truck drivers' wages for 24 years, so that the stock market can do its magic, before collecting what genuinely would be a giant windfall.
That's really hard to imagine. Missing out on the more robust income of a college graduate in your late-20s and early-30s means making sacrifices in terms of the clothes you wear, the vacations you take, the car you drive, the type of person who's likely to marry you; your dental care; your children's schooling, etc., etc.
The temptation to start plucking out money from the S&P honeypot -- in the name of having a better life -- would be fierce. Nothing wrong with that. But if that big knot of savings is used to help pay for a better standard of living at age 25, or 29, or 34, then that means settling for a zero or negative return for that segment of money going forward. Goodbye to the fabled 7.1%.
Bear in mind, too, that the stock market doesn't deliver 7.1%, year after year, with the happy predictability of a government bond. If your start year was 1999 ... and you waited 10 years to see how you were doing, you'd be looking at about a 30% drop in your portfolio. (You would have caught both the dot-com mess and the bankster mess.) http://www.multpl.com/s-p-500-historical-prices/table/by-yea...
It's easy, with hindsight, to say: "Just wait it out." But at the time, the pressure to get out of stocks is intense. And the anxiety about having bet your lifelong well-being on a market index going haywire would be excruciating.