For the record, the absolute worst case U.S. stock market modeled in the link above is actually worse than what any real-life investor has seen.

The market has never melted down to the extent that you’re left with nothing.

Literally nothing? No. But if you'd had some portion of your SS benefits tied up in say Bear Sterns, you'd be an awfully unhappy camper today. You wouldn't have lost literally everything in that investment, but damn close.

There's a reason suicides went up so much during the depression.

You left out “During periods when the market completely melts down one loses *everything*.”

Because it's not true. The market has never melted down to the extent that you're left with nothing. And we're talking retirement annuities, not a one-time withdrawal where you'd be playing the lottery.

Moreover, retirement savings is a long-term, large scale enterprise that reaps the benefits of compound interest and dollar cost averaging. During periods when the market is in decline, as it is now, one’s investment buys more shares of stock. During periods when the market is growing, as it has for most of the past thirty years, one earns dividends which are reinvested.

You left out "During periods when the market completely melts down one loses *everything*." Which is kind of the point to not tying the market to our social safety nets.

long periods where stock performance underwhelmed compared to fixed income

I'm sure that's the case. But that doesn't mean that the government wouldn't have been better off investing in stocks rather than just spending the money and relying on tax revenues to pay out said fixed income.