If it were guaranteed that the price of bitcoin would stay the same as it is today, would you still buy it? My guess is, the answer is no. But if its value lies in being a currency, price stability is essential. And if your answer is no, how is it then not just a Ponzi scheme? In other words, everyone is buying it on the hope of someone paying more for it in the future, not to use it as a currency.
So then any investment ever is a ponzi scheme according to the OP. I don't think the OP knows the definition of ponzi scheme.
Also, while the reason Bitcoin ever gained traction is because of the price volatility and its continued exponential rise, that is basically just a trick to get people to use this revolutionary tech. Once hundreds of millions or billions of people have bought it, and the market has started to mature, and volatility goes way down, people will use it because they have it already! At that point it can act as both their own personal bank account as well as an easy quick way to pay for anything (once merchant adoption expands which will happen as adoption expands). The capped supply is the economic trick to get people to buy in as an investment, and all its other features is why people will use it once they have it and the market has matured.