Bitcoin is not a Ponzi.
In a Ponzi, if there are no more buyers, it collapses.
On Bitcon exchanges, if there are no buyers, the price would remain constant.
Bitcoin is a global borderless potentially anonymous value transfer system. It uses its own units of accounting which have to be kept scarce in order to be valuable (I don't think it doesn't matter much in this context if they would have been designed to slightly inflate forever or eventually cap out at 21 million, except the capped variance surely has always been more appealing to libertarians).
If people choose to invest, all they need to do is to hold some of the 21 million units whose value is determined by supply and demand. So they act like an asset, a share. But it's a share in an openly and transparently developed system, which is quite a novelty. And it's taking away real market share from PayPal, Western Union and others, which surely are not scams or pyramid schemes either. Good related read:
https://bitcointalksearch.org/topic/if-bitcoin-was-an-ipo-168858Now that doesn't mean there isn't any risk if people choose to invest. They have to know what they're doing. There are risks. There could be a total loss. There could be a better competitor. Governments could outlaw exchanges and merchants accepting it.
If it keeps going, Bitcoin (or a successor, for that matter) will probably go through the
hype cycle, multiple times over, fractally.
But it will probably stabilize when it has reached ubiquity and mass adoption, as this is only a matter of depth. We're far from everybody on this planet having used it at least once, directly or indirectly, or at least heard of it.
There will be no point in purchasing anything since any purchase would thin out your finite investment and in the same way you couldn't possibly sell anything.
This refers to a thing called Gresham's Law. This only is valid when the weak currency that people want to get rid of (USD) is enforced by a government. If there is no such enforcement, then think again. People may want to get rid of their weak and devaluing means of exchange (corn, cattle), but merchants will prefer the strong ones (gold, silver, bitcoins).