I don't think bitcoin is a ponzi scheme. First of all, bitcoin is decentralized and this is the strongest argument in favor of the fact that it is not a ponzi. Bitcoin has very good growth prospects, you just need patience.
No doubt that bitcoin is not a ponzi scheme. It is real and trusted. Perhaps it was a ponzi scheme, then it may not be so much succeeded, and may not be still in active form. Bitcoin will remain in its origional position forever.
Why is ponzi? I know people think about volatility which make many people loss but its not reference to say its a ponzi or not.
In other trading like forex also many people loss and profit and its the same with crypto. Will you say forex is ponzi? I dont think so. Bitcoin is made by blockchain technology. The technology is the proof that bitcoin is not ponzi.