A DJIA index bought on September 3, 1929 was underwater until November 23, 1954. That's over 25 years to get back the exact amount you invested. Of course factoring in inflation, that investment had actually shrank.
The attitude that the market had nowhere to go but up, and $1 invested this year was assured to be $1 + $X ten years from now, is the exact attitude that investors in 1929 had (or 1999 for that matter).
Well, yes, but no one is suggesting that making a single large buy on one particular day - any day - and no other investments at all after that, is a sound strategy.
Considering that the time period you're talking about saw: the Dust Bowl, the rise of Fascism, a global conflict on a scale never before seen, industrial genocide, and the start of the Atomic Age and with it the threat of nuclear war and the annihilation of our species... the fact that a 20 year-old could have made the insane investment choice you've presented, and still managed to break even by the age of 45, is actually quite remarkable.
(And I'll just ignore that you've not accounted for dividends.)
How many times in 100 years was this true, besides the 2 times you mentioned? There is risk everywhere. We can only hope to take on risk for a goal after doing our due diligence to understand and mitigate the risk.
Adjusted for inflation, the SP500 had a peak in 1968, then dipped and did not return to this real level until 1992. 22 years.
The S&P 500 entered 2000 at 1425. Not until 2013 would it open the year at or above that. Adjusted for inflation, it did not hit its 2000 level until the turn of this year. It has been tanking the past few days.
The attitude that the market had nowhere to go but up, and $1 invested this year was assured to be $1 + $X ten years from now, is the exact attitude that investors in 1929 had (or 1999 for that matter).