From the SEC:
A Ponzi scheme is an investment fraud that involves the payment of purported returns to existing investors from funds contributed by new investors.
The key part there is from funds contributed by new investors.
Sticking with just Bitcoin -- I think it's a misapplication of the word "ponzi" to apply it to Bitcoin. So we're in disagreement there. Nothing about Bitcoin is a Ponzi in the slightest.
Whether Bitcoin has value or not is another question, but it's unrelated to the notion of a Ponzi scheme. My opinion is that everything that does not have obvious utility (e.g. food, water, clothing, shelter...) is really just a belief masked as value. This is true for Bitcon, securities, gold, USD, or any other fiat. Belief is central and, in of itself, is not fraudulent. It's the main precursor to store of value.
In the end, if Bitcoin loses all its value, it will not be because of a Ponzi architecture. It will happen because there is no more belief in its store of value. The same goes for Facebook, Amazon, Google, Netflix stocks (which are overvalued).
Whether Bitcoin, cryptocurrencies, or blockchain deliver on the promises of many remains to be seen. I agree there is a lot of hype about this. However, I think it's too early to make predictions on success of failure. I personally think the impact is going to be huge, it's at least 20 years out, and it's going to happen in ways we can't guess right now. Just like the Internet.
I also think governments, in the end, will have a difficult time dealing with these technologies. I don't think all governments will act uniformly -- so participants (business, investors, etc...) will play the arbitrage game and move their wealth to where its wanted.