It's constantly happening and what we see next is that the price recovers itself, so what would this be? Maybe a sustainable Ponzi scheme that is always recreating itself even stronger? Increasements and decreasements in price are part of Bitcoin's nature, and although there is speculation, there is utility for Bitcoin, so it doesn't seem a Ponzi.
The price recovers itself because in my opinion we haven't seen a real hard drop yet. Even the "big crash" that happened in 2013 was due to a platform hack, so it joins the category of "heavy weights fluctuations".
As I said, we are still very early in the curve of acceptance. The more users get in, the easier it is for bitcoin to recovers. My fear is : what happens when the curve of acceptance is done ? What happens when nobody new comes into the market ? The price will be unpredictable.
The huge advantages of bitcoin are about being used as a currency (security, transaction immutability, privacy...), otherwise its value lays on nothing (or maybe the physical electricity that miners invested to secure transactions and generate new bitcoins). That's why, if nobody use it as a proper currency, it IS a Ponzi : you invest, sell at higher price, the one who bought does the same, etc etc... until there is nobody left to buy it, no new users.
I didn't say that it was in reality because, as Mometaskers, there is some actors that consider using it as money, but we need heavy weights AND user support. HODLing will become dumb at some point.
Well, the 2013 big crash was a real hard drop anyway. On that time people could stop using Bitcoin, but they didn't. If a hard drop happen again people will remember that day and won't fear to continue using Bitcoins.
I believe when nobody new comes to Crypto-Currency world, it will be because all the world's population or a big parcel of this population will be already using it and many different businesses will be already accepting Bitcoin or/and other Crypto-Currencies as payment methods. There will be a use for this, without speculation we can surely see a drop in price, but not enough to "Kill" the currency.
And don't forget gambling industry, it's helping to bring more people into Bitcoins, and not only for the speculative reason, but the recreational one. It's an industry that moves really huge amounts of money on the world.