Couple things:

One, the government does not promise to pay you back whatever you pay into Social Security, with interest, and it never has.  As I have already noted, you can get back nothing, or you can get back well more than you paid in, merely by living a shorter or longer life.  If that strikes you as unfair or wrong, fine-- just don't say that the government promised you your money back with interest, because they didn't.

Also, Social Security is not a Ponzi scheme.   As expanded upon in the linked article written by a man who literally "wrote the book" on Ponzi schemes, Social Security bears only superficial similarities to and lacks several critical signature attributes of real Ponzi schemes.  Most significantly:

1. Ponzi schemes entail deception, whereas Social Security is exactly as advertiized-- current workers pay into the system, and that money is transferred to current retirees.

2. Ponzi schemes are inherently unsustainable, whereas Social Security can be-- it only requires that politicians adjust taxes and benefits to account for a shrinking workforce and an expanding population of seniors.  In fact, if demographics were constant, Social Security could continue indefinitely without any change at all to taxes or benefits at all.

3. Ponzi schemes are designed to enrich the originator of the scheme.  Social Security was designed to "enrich" old and disabled former workers, who were often destitute before Social Security was enacted.