Is Bitcoin a Ponzi? How exactly?
And that greater fool must in turn convince an even greater fool, etc, etc
It's not a Ponzi, but it has similar dynamics to a Ponzi in that you can only gain money through other people losing money.
Bad price stability, slow transactions, high fees under any amount of load, bad privacy, terrible adoption, etc, etc
It's a garbage currency - fees are too high, transfers too slow. It's a garbage technology that encourages wanton energy wastage. Its only "value" is as an investment.
Or even about modern fiat currencies. Just look at the Turkish Lira (~343% vs EUR / 5y), Argentinian Peso (~758% vs EUR / 5y) or a dozen other currencies that lost insane amounts of value over the last years [1], mostly because of political misgovernance and resulting economic devastation.
Any investment is supposed to have a return that exceeds inflation or some other reasonable index, eventually in the eyes of some kind of reasonable investor.
If it doesn't meet that criteria then it is more likely a social phenomenon interchangeable with scam instead of a valid investment, i.e. a pump and dump OTC stock is an investment being involved in a scam if it has acquired actual rights or business that have a small probability of paying off significantly better than inflation that's not easy to disprove with basic research. It is a pure scam if the underlying company is only on paper or generally incapable of actually doing anything it claims because it was created or is pumped with just enough cash to continue to operate purely for the purpose of having a stock but is incapable of ever making a real profit.
Some coins may be different than what I described as a scam, but if you accept that only holding value remains as a probable use of BTC then any suggestion of it as a valid investment or holder of value is a scam.