There is no good reason for anyone to think they are investing for a future return in SS. ...
That may be true, but most people of modest means do consider SS an important part of their "savings" for their retirement years and they are very aware of the fact they are paying the SS security tax ("investing" in their future, via SS).
How does the average SS contributor (an investor from their mistaken POV) differ for the investor in Madoff's scheme? Both contributed money in the hope of getting more back later. That money was not actually invested to grow for them, (even though they thought it was).
In both cases, the current contributions were used to pay those currently collecting. If SS is not a Ponzi scheme, then neither was Madoff's plan. There is no difference in how these two plans operate, except SS is legal and compulsory and participation in Madoff's scheme was voluntary.
In both case, it is certain that someday the then current contributors will get screwed as their contributions will not even pay the current expenses. In the case of SS as it currently stands, that day is in 2024, as I recall, but SS will be modified to change that "go broke" date; however, the "go broke" date cannot be eliminated unless the increasingly geriatric distribution of the population is eliminated. (There is a thread about euthanasia of the old where this is being discussed.)
SS will probably be the longest running Ponzi scheme in human history, but it still is a Ponzi scheme. When it collapses, the last in contributors will get screwed, just as in any other Ponzi scheme. As an "early investor", I will get back much more than I put in (my compulsory investment).
Actually, SS was a significant reason why I retired at the end of 1993 - Despite years in school, I had my "40 quarters" at the max rate contributed, and the max contribution for 1994 took a big percentage step up. As my older max rate contributions would be stepped up with inflation, the payout I received later would not be significantly greater but my total contributions to SS would be if I continued to work, so I retired early. (My frugal nature and other investments with the resulting saving made that financially easy.)
There was a more important factor in my decision to retire early - A Brazilian lady, a beautiful university professor, I briefly met during a Mexican vacation. - After some letters and several exchanged visits, I decided I wanted to go and live with her permanently in her Sao Paulo apartment. I am starting my 16th year of living with her there. A couple of years ago, we got officially married. - Not good to rush into marriage, but we are getting older; however, but both are very healthy still. I look forward to collecting SS for 25 years more, at least. (She owns her apartment; I pay only for our non-party food, which is cheap as little meat is eaten, so I can live on only my SS income in Brazil.)