It’s not a Ponzi scheme because withdrawals and deposits are scheduled and mandatory. You don’t get into a situation where investors lose faith and ask for money that you don’t have because that’s not allowed. You get your money when you reach the requisite age. You also don’t get into a situation where you run out of investors because it’s a mandatory payroll tax.
Essentially the only issue facing social security is the fact that since 1974, wage growth has become decoupled from productivity gains, meaning the payroll tax that funds social security captures a smaller proportion of business revenue over time - because businesses spend less of their revenue on payroll than they used to.
Social security doomerism of the type on display here is a tacit acceptance of conservative propaganda on the subject. The government is fully capable of indefinitely maintaining a pension fund, we just have to stop accepting the lie that it isn’t.