If new members join the scheme at the rate that old members die, and the average member expects to get out what they put in, then it is a very sustainable Ponzi

If new members join the scheme at the rate that old members die, and the average member expects to get out what they put in, then it is a very sustainable Ponzi scheme. In theory you'd have the problem of inflation if each generation put all their contributions into a dedicated fund which was later used to pay their expenses in old age, but fortunately the current contributors can pay for the current beneficiaries, which mitigates that problem.