We call Social Security a Ponzi Scheme despite the fact that it's not legally possible to be the last guy to receive a Social Security check. SS is funded by a tax on wages, so as long as somebody's making wages SS Beneficiaries will get checks. It's possible (and actually pretty likely) that the returns of this generation of small families will be lower then the return to previous generations who routinely had three kids, because three kids = three salaries to tax, but nobody is gonna lose all their money.
That's actually funny. I hadn't thought of it but social security is a Ponzi Scheme, just a government-mandated one. It's not that you put money into the system and you get the money back out when you retire. Rather, you put in money to pay people who have already put money in, and when you retire you get paid by new people putting money in. The reason a Ponzi Scheme collapses is that people stop putting money into it. The difference with SS isn't that it isn't a Ponzi Scheme, but rather that you're legally required to put money into it. As such you're right, it'll never collapse - unless the law changes, which it might. Whichever way the law changes, either someone will end up being screwed, or the gov will have to pay them what they predict they would have gotten from a different source.
So essentially SS is a ponzi scheme where you are extremely unlikely to lose your money, while bitcoin is not a ponzi scheme where you are perhaps likely to lose your money.