If people think bitcoin will become more valuable, because they suspect future people will think it will become valuable, because future people think it will become more valuable, because future people think it will become more valuable, then eventually you run out of people and it collapses.

We can visualize this as the last person on earth not investing in bitcoin because there is nobody after him left to invest in bitcoin to make the price rise more. And by that same logic, the previous-to-last person wouldn't invest either because he knows the last person won't invest, and by that same logic, the previous-to-previous-to-last person wouldn't invest, etc etc etc. At some point the pyramid simply stops because it's used up all the fertile soil, and the growth itself was the only value creation. It stops ways before the last person on earth, the moment the market is saturated the growth stops and the entire thing collapses.

But if people think bitcoin will become more valuable, because they suspect it will be useful as a decentralized currency to more and more people, then it's not a pyramid scheme in the same way. Even the last person on earth would have ample reason to start using bitcoin, it's just a convenient way to be able to pay for goods and services in any context. Early adopters do get a big payout, but that's because they were early in on something that's valuable on it's own.

Obviously these days the later scenario looks less and less likely as Bitcoin's everyday adoption has slowed down, while Bitcoin's role as a growth speculation seems to have taken over.