Completely untenable when the generation retiring has also taken the lions share of property wealth, imposed student debt upon and consistently voted against the interests of the generation paying for their retirement.
There is another problem: many people go into decline as they get older. My company retires some people at 40 with a full pension because most people physically cannot do the job anymore (I'm not sure if they are able to do much else after that). Airpline pilots are required to retire at 60 (I don't remember the exact age) because we are sure none are very far into mental decline even though a few could work for 30 more years.
However, the current system is quite far from that. The money you pay in goes nearly immediately to existing retirees with a moderate reserve of capital that is slowly decreasing as more money exits the system than enters it. This system cannot pay current retirees without the investment from new users, and that's what makes it a ponzi.
Until we invent self-replicating physical capital (i.e. a Santa Claus machine that can make other Santa Claus machines), this will always be true.
No, they are not. “A ponzi” is a fraud. Some of the schemes you mention are not sustainable under certain hypotheses, but none of them have the deceptive and fraudulent aspects of Ponzi schemes, which is important. These are strategies, that might work out or that might not, but in any case they are not tools for personal enrichment through deception.
So what?
This doesn't need to be a ponzi scheme though. Even if population size is stable, if people on average work longer than they are retired, they have to pay less than they receive, all things equal. A problem here has been that people get increasingly older, so they spend a larger percentage of their life being retired, making the ratio of payers vs. receivers worse. The most obvious solution to this is raising the age of retirement, but that's always politically difficult and somehow seen as cruel.