Its a bit pedantic, but please don't say bitcoin when you mean cryptocurrency, as bitcoin is a pretty bad one.
And that's exactly the whole point, and is the very reason it's working.
> a pyramid scheme
Then every other successful investment is a pyramid scheme. Gold is a pyramid scheme! Storing food for catastrophes is a pyramid scheme! Paintings are a pyramid scheme!
What makes bitcoin into kind of a pyramid scheme is that right now a big part of the reward for miners is the 25 bitcoin you get from mining a block. Only a very small fraction comes from the transaction fees. The only way to keep bitcoin stable is by having a constant supply of new users buying into bitcoin by acquiring those fresh bitcoins from the miners.