Bitcoin's original goals and a ponzi scheme both require collective belief in a fiction, but they are distinct fictions.

Bitcoin requires its users collectively believe it has value and others will accept it for some value - this is necessary in order for mining to be profitable thus hashing power be dedicated to protect the blockchain.

A ponzi scheme requires that more people will be buying in at increasingly higher prices.

Notice the former does not assume anything about constantly rising prices. The original vision of Bitcoin actually works better if the price itself remains stable (not rising).