Needing new buyers for price to increase is not the definition of a Ponzi scheme. Because that is true for any speculative asset due to the price setting mechanism. Your argument entails that the price of Bitcoin is highly inflated over its fundamental value, i.e. that it is a speculative bubble. Speculative bubbles are very different from Ponzi schemes. A Ponzi scheme has the following definition according to the SEC:

>“A Ponzi scheme is an invest­ment fraud that pays existing investors with funds collected from new investors. Ponzi scheme organizers often promise to invest your money and generate high returns with little or no risk. But in many Ponzi schemes, the fraud­sters do not invest the money. Instead, they use it to pay those who invested earlier and may keep some for themselves."

Ponzi schemes require a central fraudster to pass on money directly from old to new investors with the underlying asset obfuscatedly held on promise by the fraudster.

With cryptocurrency the buyer has control of the commodity, you actually have control of the crypto, there is no promise of a consistent return with little risk, there is no central entity defrauding targets with promises that hold the asset in reserve passing on money directly from new to old investors.

What we do have is a speculative bubble which will no doubt be devastating for new investors when it collapses. But the definition of a Ponzi scheme is not met.