Bitcoin is a giant Ponzi scheme!"
Almost 15 years after the introduction of the Bitcoin's whitepaper, I still hear this declaration frequently, and usually with a conviction inversely proportional to the price of Bitcoin. I am sure you have seen various versions of this claim by now, but here is one of my all-time favorites.
Allow me to elaborate why saying "Bitcoin is a Ponzi scheme" means you have no understanding of what a Bitcoin is, but perhaps more importantly, neither what a Ponzi scheme is.
What is a Ponzi?
First, let's revisit what makes a Ponzi scheme from the SEC itself:
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.
Bitcoin transactions are transparent and verifiable
If I buy a Bitcion from you and receive a Bitcoin at the price agreed, where is the Ponzi? This is not a discussion on what the price of a Bitcoin should be. In a Ponzi scheme, the promised returns are not realized and new funds are funnelled to the previous investors to sustain the returns, or outright stolen. A Ponzi scheme's central characteristic is that the funds available are not sufficient for the operations of the scheme and hence the need for constant flow of new money to repay older claims.
An exchange made for Bitcoin is on the public blockchain and anyone can independently verify that promised Bitcoin left your wallet and arrived in mine. There is absolutely no ambiguity in a straight-forward Bitcoin transfer about the parties involved and the amount that changed on the network.
A Bitcoin exchanged for any other currency is no different that exchanging USD for EUR. You may have a view on how under or overvalued EURUSD cross might be, but even no matter how undervalued you think the US dollar is versus the Euro doesn't mean that there is a Ponzi scheme involved. It is an equal exchange of two assets, not an investment fund run by a central party.
Disagreeing over the fair market value is what makes a market of buyers and sellers. Otherwise, why would there be such a high volume of transactions every minute of every day far exceeding the value of total economic output?
Greater fool theory
To be a bit generous, I presume when you hear someone say Bitcoin is a Ponzi scheme, they are trying a hyperbolic version of the greater fool theory. Maybe. I have generally found the "Bitcoin is a Ponzi scheme" to be a bad-faith argument with little understanding of crypto, or Ponzi schemes for that matter. Whether BTC at 55k is a buy or sell has nothing to do with a Ponzi scheme argument.
Bitcoin is many things. There are a number of valid criticisms. A Ponzi scheme, however, is certainly not one of them.