Well yeah, it's not exactly a Ponzi scheme per se, but it is a lot like oil and other natural resources, but they're a resource and not a currency. It is quite a lot like a natural occurring Ponzi scheme though. I don't anyone has actually explained this yet, the core point I'm getting at is if the supply is inherently built to continually diminish, then wouldn't the price continually increase, and that in itself is can't sustain a stable currency and the people who invested early will never be at risk because their asset will be guaranteed to continually appreciate in value
Good, as you note, it is not a ponzi scheme. Ponzi's have a central actor who uses new investor money to pay old investors. What you've described above is any scarce commodity, e.g. land. As the second poster to the thread pointed out, a simple review of the definition of Ponzi would illustrate the difference.
Financial bubbles are possible with any financial instrument. Bubbles do not equal ponzi's, that's why they are called bubbles, regardless of what Schiller kinda sorta says.