Social Security never had to be a Ponzi scheme. But when it was drawn up, very few people lived long enough to make a significant withdrawal of the funds that had been deposited. Retirement age was arbitrarily set at 65, when average life expectancy was less than 60 years.
The supposed “Lockbox funding” has been raided time and time again, for short-time contingencies, supposedly to be paid back in the future, but to date, none of the pilfered funds has ever been restored. All there is in the “lockbox” are a bunch of IOU’s, now worth about as much as the paper upon which they were printed, or in today’s digital world, the computer storage space on a server somewhere.
The financial industry at the time warned the FDR Ivy League geniuses it was not sustainable. However, by the time that happened they would dead, and it would be someone else’s problem.
That started with LBJ and has contiued every since. It was during the Vietnam war and the slogan was "Guns and Butter." What that really meant we will fund the war by raiding the social security trust fund and hand out largess to the public at the same time. They stole the money. It should be noted both Democrats and Republicans were in this together!