Surely that's only true if you're trying to use BTC as an investment to make capital gains in USD with? If your BTC stays within the BTC community and is traded directly for goods/services without conversion to USD then there's no such requirement.
Now, as it stands the vast majority of transactions are actually basing BTC's value on USD. I think this is a misstep, but perhaps a necessary one.
What I'm saying is: you're right as long as people see BTC as a vehicle for USD gains. But the grand idea is to replace USD entirely. When we start seeing people routinely selling things for fixed BTC amounts, regardless of the 'exchange rate', then bitcoin will have succeeded.
At present it's largely just a proxy for USD, as Warren Buffet has noted. But this is just the first stage.
A pyramid scheme is where each person who buys in sells parts of their own share to the next mark, usually kicking up a fee to the person who sold their share to them. The returns at the top come from the exponential growth of fees from the bottom.
Bitcoin is only really superficially like either of them, mostly because there is no direct kickback aspect of it. If it's any kind of scam it's an asset price bubble.