Ponzi, or no Ponzi, it's still an illusion: let's assume that we all generate enough money saved in those funds during our productive years to sustain ourselves in senior periods, even counting in the almighty inflation. The problem is it's not money you need when old, and frail, you want somebody doing work for that money. So the whole scheme still requires enough young workers entering the market, and them be willing to perpetuate it. Which is kinda not guaranteed if each gen is smaller then the previous one.