Critics on both sides like to call it a Ponzi scheme, but it's really much more sophisticated than that. A Ponzi schemer couldn't just up and change the eligibility age, level of payout, or amount of required contribution to keep the whole ship floating. It is really a lot more robust (Rightly or wrongly, YMMV) than people attacking it understand, or want to admit even if they do understand the structure. Unlike a Ponzi scheme, it's not a game with permanently-fixed rules, and all the dire projections are based on the false assumption that today's snapshot of its operating parameters are fixed forever.