Are you aware of the historical return rate for the S&P 500? It's between 10 and 10.5% over a period of 98 years.

Sure, it’s great to predict with history. Let me try:

You go to work in 1928. You invest in an index fund (which didn’t exist at the time, but oh well). Then you lose your job in 1930 or 31 or 32. You don’t work for most of the decade, so no investing. You go into the military during the 40’s, so no contributions are minimal, so your investing is minimal. Your savings, pitiful as they are, sit in your index fund for 24 years before they come back to “even” (1953). That’s most of your investing life, and you have a scant few years to start building again.

index funds with standard downside protection

I am unaware of any “standard downside protection” that comes for free. So you are going to pay for “insurance” against the downside? Novel concept, I guess.

There was always a structural problem of a population that had never saved anything during their working years and as such SS is structured to be a transfer payment.

That is correct. Prior to SS about half of elders lived in poverty. If SS went away today, that number would be almost exactly equivalent, based on surveys taken of recipients’ financial conditions. We can conclude therefore that it has not changed savings behavior but it has changed outcome of life. That should be a pretty good result, I would think, except for the very last cohort when the Republic falls and there is no one left to pay. Of course the same thing will happen with roads, electrical infrastructure, and all the rest, but that is decades away, I hope, unless we keep electing Republicans.