Future generations (and the future is now), in my opinion, have no moral obligation to perpetuate the Social Security Ponzi Scheme - and that includes paying out benefits to people who "paid into it."
If the money had gone into an actual trust fund that would be one thing, but it didn't - it went into a "wink, wink, nudge, nudge" pretend trust fund while the actual money was actually spent. No matter what euphemism they tagged it with, the reality is that it was (and still is) a tax being used to pay for current expenditures... not something being saved for the future. This is one of the great dangers of collectivism - no sane individual would (absent coercion) participate in such a scam. Considering the behavior of government over time, it would be like investing your life's savings in an alcoholic's drinking binges. It's a scam, it is and has been well-reported as being a scam, the politicians running it certainly know it's a scam, but enough people decided to cover their ears, close their eyes, shut up and go along with it to allow it to perpetuate.
The question becomes: are future generations (that is to say, us) beholden to this scam? Are we morally obliged to pay real returns on their pretend investments? I would say no, we're not.
If people were to raise a great hue and cry at this (and they certainly would), we could simply issue their payments in Monopoly Money (the game kind; not a trite reference to fiat currency) and assure them that these payments are every bit as real as the investments they made.