I don’t think social security was supposed to be a pyramid scheme. It was supposed to be drawing out what had been paid in.
From the start, funds paid in by others were used to pay then-current beneficiaries. That is the essence of a pyramid scheme.
It was supposed to be that only the exceptionally long-lived were around to draw benefits, which is why it could have been solvent in conception. We live so much longer now that people have come to expect it to support them for the last 25 years of their lives—not possible with the amounts paid in.