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.
This is where your wrong because everything in this Earth depreciate/appreciate examples are precious metal, oil, food, even FIAT money may go up or down. So why not crypto. If you think because its price goes up then everything can be considered a ponzi scheme.
You also need to rethink your definition of ponzi scheme because it is way off if you compare it to the bitcoin/clock chain system.