You either don't understand how a Ponzi scheme works or how SS works or both.

The fundamental problem with a Ponzi scheme is that there's no actual business that's generating wealth.

But with SS there is real wealth generation, the entire US workforce. In a sense, the US workforce is the "business" that generates wealth for the retired. They do not (normally) put your SS payments in a bank for 40 years, they give them directly to retirees.

So literally the only way for SS to not have money to pay retirees something is if everyone is unemployed.

A ponzi scheme can run out of investors to buy in.

Do you think the US can reach 100% unemployment?

It is true that benefits can drop if there aren't enough workers making enough money, but the only way SS can completely fail is for no one to be employed. It should be obvious why that isn't going to happen.