Small addition: for something to become a currency, it must have value and market acceptance. The value is coming only from the speculative desire to hold money

Small addition: for something to become a currency, it must have value and market acceptance. The value is coming only from the speculative desire to hold money (either expecting it either to grow, or at least to outperform alternatives, like Argentinians holding dollars to protect against peso inflation). In case of Bitcoin, the amount of acceptance is reflected in the price because the more people want to hold it, the higher price they will have to bid. Therefore, price tells everyone that 1) bitcoin has value and 2) bitcoin has growing acceptance. Only after that it can be used as a currency. TL;DR: Speculative investment is the only way for an asset to have value and thus become useful currency.