Bitcoin is a pyramid scheme because it only keeps its value as long as people are constantly buying it. If no one wants to buy it, the value of any amount of bitcoin is zero. This is why people who have bitcoin are trying to convince anyone else to keep buying.
any currency is initially a bank's promissory notes, and then a promise to exchange the paper for some kind of labor. As a person who has experienced at least one default in his life and whose entire toilet is covered with USSR money, I can say that in this regard, no currency is different from bitcoin.