If you look at the classic definition of a ponzi scheme, bitcoin doesn't fit, though central bank fiat currencies do. See macro economist Lyn Alden's excellent article for the full explanation: https://www.lynalden.com/bitcoin-ponzi-scheme/
I see a key difference: we choose to participate in a Ponzi scheme, whereas the central bank fiat system is one we are forced to participate in, just by means of being born in a particular country. There’s no choice involved, being backed by the full force and legal monopoly of the state. That’s a key difference many tend to gloss over when pursuing this characterization.
The details may be different but the active ingredient is fraud.