Ok maybe using the term Ponzi is a bit too much. Certainly saying Ponzi like might imply a lot more. But no one's disagreeing that bitcoin, with its finite supply is a speculative goldmine, just like oil, housing and other natural resources, except for that it does not have any inherent value - that it can't really be used for anything other than trade with someone else who also deems it valuable. Doesn't its speculative properties render it obsolete as an viable form of currency for trade, which is it's supposed primary purpose. Or is bitcoin made for speculative purposes? Satoshi made something else for people to speculate on, just like those dutch tulips. Forget Ponzi, think man made speculative bubble
But because a lot of people using Bitcoin, Bitcoin has value. If Bitcoin loose userbase, it loose value. The same with any natural resource, once some natural resource is not used anymore, it loose value.