My personal theory is that cryptocurrency is not the bubble, but rather the needle that will burst the fiat money bubble. Trough history we have always had an easy way to attribute value to things without it. Those who believe that fiat money is the only true way often attribute their opinion to the fact that fiat money is substainsable because of its centralized nature and its history of being used as a represantation of value
Right now it is (for the most part, at least)
Bitcoin and other cryptocurrencies fit well into the definition of a financial bubble. I could even claim that Bitcoin is a sort of tulipomania. Just like tulips, bitcoins are useful for some purpose, namely, for transferring value around the world while completely bypassing centralized (read controlled) entities like banks, but Bitcoin current price is by no means determined by this utility. Most of Bitcoin value today comes from sheer speculation. Whether Bitcoin bubble is going to pop eventually or its real use as a currency will finally match its use as a speculative vehicle is not clear as of yet