I might call it a public pension system whose books are slightly out of balance but where the imbalance could be corrected with a relatively simple fix or two.  I could say that but shouting “Ponzi scheme!” would likely get me more attention.

Dean Baker, Social Security Does Not Redistribute From Young to Old, It Is a Public Pension System:

One of the most pernicious myths of the Fix the Debt Gang and other Peter Peterson type outfits is that Social Security redistributes money from the young to the old. This is bizarre because people pay for their benefits with the taxes they contribute during their working lifetimes. In fact, the average return current beneficiaries receive is not especially high (less than 2.0 percent real).

If workers contributed the same amount to a privately managed pension fund and then collected an annuity in their retirement no one would call it a redistribution from young to old. It hard to see how it becomes a generational redistribution because Social Security is run by the government…