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
Bitcoin has had very large drops in its price and this risk is still present.
The number of Bitcoins in existence never inherently decreases (though it will if people lose their Bitcoins), but the rate at which it increases will slow down over time.
That Bitcoin is supposedly guaranteed to increase in value over time, as you say, is a self-defeating prophecy. If anything is guaranteed to increase in value over time, people will speculate in it right now, which will drive up the price right now, which means it is already at the point where people expect it to be in the future. The role speculation plays in the value of Bitcoin thus dwarfs the effect of reductions in the rate of Bitcoin creation.