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.