This also applies to private retirement investments. My comfort in retirement is contingent on the economy, and hence, the stock market, growing. If the 'ponzi scheme' of limitless growth collapses, I'm not going to be any better off than the pensioner.
I'm not sure that any public pension systems in any way satisfy the definition of a Ponzi scheme. While public pension systems may be flawed, in no way are they fraudulent or otherwise non-transparent at the very least in democratic regimes.