And how IS fiat a Ponzi scheme? You don't buy into the US dollar expecting your "share" of dollars to go up. You just use it to pay for stuff.
But the US could end social security tomorrow and as long as it still required taxes and tariffs in the form of dollars, the dollar would still have value.
USD and other fiats are doing the same. Just at a much slower rate.
Edit:
It's basically just the Cantillion Effect[1] by another name.