Energy costs alone will sink bitcoin longterm leaving only initial investors with any true value. It's the very definition of a Ponzi scheme due to this unsustainablity.
That's not what a Ponzi scheme is. In a Ponzi, earlier investors are paid from what newer investors put in. While there have been Ponzi schemes that used Bitcoin or Ethereum or what-the-fuck-ever, they themselves aren't Ponzis – they're essentially commodities. They're stupid, wasteful commodities, but what else is new?
With your definition, stocks and commodities are Ponzi schemes too. It's clearly nonsense.
And no, before anybody starts getting any ideas, in case it's not already blindingly obvious I'm not a "crypto fan" by any means. There's some technologies and ideas there that might be useful in the long run, but 99% of the field is bullshit
If this is the definition of a ponzi scheme literally every currency could be considered a ponzi scheme. Why is the US dollar worth anything? Because enough people agree it is so if they stop agreeing it becomes worthless pieces of paper, or a ponzi scheme.