It's like this..... I found a calculator on the internet that tells you what a dollar invested in the S&P 500 would be worth today if it had been invested in 19XX or 20XX (insert the year for XX) so I took my wife's earning report (what she's paid into SS) and calculated how much each year's payments would be worth at the end of 2016 (as far as calculator calculated when I did it). When you summed all that up she would have had $992,000 and that's just her part (remember her employer had to match that).It's not hard to see that if she had gotten all the money that she and her employer had paid in she would have well over 2 million dollars today and even at the paltry 2% that CDs pay today she would get a lot more that she would get if she filed for SS. Using the old rule of thumb for investments where you can take out 4% the first year and then adjust it for in future years you can see that she would get $80,000 the first year if figured on 2 million and that is more than 3 times she would get today if she filed for SS so yes it is a bad investment for the average person.
I realize there's a type of insurance built into SS so the people that use that do come out way ahead but the average person today will never get even get the interest off what they put in.
There are major bend points in the SS system....I once did an analysis of this, but can't recall the details.... other than the best returns go to the lowest income workers by far.... with the caveat that they don't die early. Now that's the tricky part.... as we know higher income levels are positively correlated with longer life expectancies... for a multitude of reasons.
Like any pension system..... we don't really know ones true ROI until after they die.... and the benefits stop (or are curtailed, or stop when the spouse dies, etc....)
It could end up a terrible, terrible, deal... in that they die 1 day after being eligible for benefits.... or it could end up being just ok if they end up living an average life expectancy.... to possibly pretty decent if they live to be 104.
Comparing its internal rate of return to the SP500 isn't a very proper comparison (though certainly frustrating in hindsight)..... because it's not an equivalent risk adjusted return. There is no market risk in the SS system. The proper comparison would be if all the SS contributions were invested in a basket of Treasury bonds (which is what the SS trust fund does) . This makes the deal quite a bit less "ugly".
I'd personally (as it seems you would).... be willing to take the market risk myself over a working lifetime.... and would happily sign up for this idea. But my years of interacting with the general public on their finances in different manners has lead me to believe that this would be a disaster for most.