Social Security is a classic Ponzi structure. If you can't see that you're deliberately not looking. Ponzi schemes always collapse, and the fallout is never pretty.
It's a Ponzi scheme in that the money isn't being invested. The money isn't working and earning a return. That's why people suggest private pensions as an alternative to social security as it can put money to work. And there's skin in the game as it's not a Pension fund board member investing tens of thousands of members savings with a single decision, it's you deciding your own future and the level of risk you're willing to take on
A fuller definition would be that Social Security is predicated on a growing working age population.
Which historically, has been a valid assumption.