I'm not sure that you understand how either cryptocurrencies or Ponzi schemes work.
Any "currency" in use by any major government today is fiat and is based on the public's perception that it's worth something. That dollar bill in your pocket is a piece of (admittedly good quality) paper with George Washington's face on it. The money in your bank account is one's and zero's. It's all worth what we agree that it's worth, but it's not valuable unless we agree that it is.
Of course, the same can be said for precious metals, diamonds (a heavily manipulated market), or any commodity really. Gold has commercial uses, but does it have practical uses for you or I? No. It looks nice and it's hard to extract from the environment. Beyond that, we agree that it's worth something, but only because of the context. If you were out in a desert for a few days without food or water and I gave you the choice between a $1000 gold ingot, $10,000 in dollar bills, a $300,000 house, and a $1 bottle of water, which would you pick first?
The fact that you believe that an unstable currency should not be invested in hints that you don't have a firm grasp of investing in currency. Money is made by buying low and selling high. If there are no lows and no highs, how does one profit? Discounting obvious garbage coins that will never go anywhere, some people look at those price oscillations and get scared. They shouldn't invest in this market. Others look at those price oscillations, get an idea of where the peaks and dips are, and then set their buy and sell limits appropriately. The software handles the rest, with panic buyers and sellers providing the income.
Somewhat ironically, if there were no inexperienced or uneducated investors playing with crypto, there would be no whales manipulating the prices, the prices would remain stable, cryptos would make more sense as actual currency, and investment would stop. It's a very interesting market.