On the surface Bitcoin is a payment network, that isn't all there is though. If Bitcoin or the other cryptocurrencies didn't at least have some value or perceived value to its users, then they wouldn't gain adoption. This usefulness doesn't negate that it has a Ponzi scheme structure to it. In a traditional Ponzi scheme, the value is the big returns you're getting - being fed by the next investor in line to be scammed. That only works when the Ponzi artist targets people with substantial amounts of money - however Bitcoin now enables that for very small transactions, globally. That Bitcoin has value outside of the Ponzi aspect, doesn't negate that it's a Ponzi structure.
A Ponzi scheme is only worthless and useless once it ends because the issuer decides to run off with a chunk of money; that likely should easily equate to people selling off their Bitcoin to eager investors who see this stock-market like system that if they get in 'early' on - or at a low enough price - they can make money selling back out.
That Bitcoin is speculation, a brand, and useful in some ways - that is why it can be sustained ... because the network of speculators/investors is global, and people with $100 or people with $10s of millions that want to gamble can join in on.
The 'single issuer' could also be considered the Bitcoin blockchain itself.
Years ago I met someone who sold off $20MM+ of Bitcoin, bought real estate for rentals, then bought more Bitcoin when low to rinse and repeat. How many people are doing this which keeps Bitcoin going, while slowly increasing the price? It could easily be 50%+ of money, of course everyone with Bitcoin is incentivized to increase adoption - so their Bitcoin doesn't become worthless, and so in fact it becomes worth 100x.
None of what you say negates the fact that it is a Ponzi. Plenty of Madoff's customers did the same things--they were "lucky" in the sense that they weren't at the last layer of the Ponzi. Someone will be. There was research done about Ponzi schemes and I don't remember all the details except that none get past 11 levels because it would require more people to invest than exist on the planet.
Ponzi systems end. A speculative asset might drop hard, but then it stabilises. There is simply no reason to stop using or holding BTC when the price increase stops.
