With regards to "it's like a ponzi scheme". I completely see what you're saying. But shouldn't it look more like a population pyramid? Where, yes, the word pyramid is in the name, but generally a mature nation won't be top-heavy (except for the baby Boomer issue but that's kind of transient, no?)
My sense is that it should work perpetually so long as a nation doesn't see massive population decline and employment remains reasonably consistent. And in years/decades when there are imbalances, the government can step in and prop it up through other means rather than watching a private corporation file bankruptcy.
I feel like maybe we're judging the long-term efficacy of such a system too early. Right now we're dealing with a one-two punch: baby boomers are retiring and people are living longer. But neither of those are going to increase perpetually.