Do you know the definition of Ponzi scheme? Because I don't think that term means what you think it means.
Bitcoin is many things, but it is as much of a Ponzi scheme as gold, real estate, or stock speculations. ie. not a Ponzi scheme at all.
While one can argue that Bitcoin is a scam (and most definitely a bubble), it does not fit the formal definition of a ponzi scheme.
>>A Ponzi scheme is an investment fraud that involves the payment of purported returns to existing investors from funds contributed by new investors. Ponzi scheme organizers often solicit new investors by promising to invest funds in opportunities claimed to generate high returns with little or no risk. In many Ponzi schemes, the fraudsters focus on attracting new money to make promised payments to earlier-stage investors and to use for personal expenses, instead of engaging in any legitimate investment activity.
The key point here is the "solicit new investors by promising to invest funds in opportunities claimed to generate high returns" section. In a normal Ponzi Scheme, the previous investors would attempt to guarantee newcomers that profit is certain.
In comparison, Bitcoin promises no such thing. While it is true that the profit of previous investors (or speculators) do indeed come from newcomers, the newcomers are not promised anything beyond their belief that the price will continue to rise.
This key difference makes the Bitcoin phenomenal a 'Bubble', not a 'Ponzi Scheme'.