We'll know if Bitcoin is a Ponzi when the last miner is powered down and the last slice of pizza is eaten.
That would seem to indicate a basic lack of understanding of the bitcoin protocol. If the miners shut down completely, there's nothing to confirm transactions. That doesn't make it a ponzi anymore than having computers store your bank account balance makes earning money at your job a ponzi.
I said nothing about the bitcoin protocol. Having the miners cease their function is the only analog in Bitcoin to a Mr. Ponzi ceasing operations. I also don't have any reason to believe there is any probability that the Bitcoin network will cease for many years.