Bitcoin is a decentralized currency, it's very normal to have high volatility, it's part of the decentralization nature. If you don't want high volatility you can use fiat, but don't call bitcoin a ponzi scheme, because it's not.
In a ponzi scheme you have nothing else than money being distributed among people, there isn't a product behind it. Bitcoin is the product, it's the currency, it has an usage and for this simple reason it can't be considered a ponzi scheme already. If you want to think it's foolish, that is fine, it's your own decision to think what you want.