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 (real 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
There's a major difference between tulips and Bitcoin. Tulips were not a very good money. Bitcoin is designed to be a money, with anonymity and security built into an electronic architecture, in addition to the usual monetary properties like durability, divisibility, etc.
Anyone can claim anything is a bubble, really. (Except for items that you can use directly and are priced only for that value -- these will never fulfill the vital role of money.) Any claim to wealth (that is transferable) must rely on the confidence of the crowd -- a network effect that says, since other people think it has value, I think it has value. When that network effect collapses, the claim's value is gone.
The main problem with state-issued money is that, while it enjoys powerful support by the state-bank alliance, all individual members of the elites have the incentives to destabilize their own money, by issuing too much money, by borrowing too much, by using deception to prop up their system temporarily, etc. -- all the history we know. So, fiat money, almost by definition, must collapse. The very strongest ones in history retain a small fraction of their purchasing power.
So, after 2000, with all the problems suffered by the state-bank system, it's not surprising to see investors looking to non-state monies, gold and Bitcoin, to store part of their wealth.
You can call it speculation, but under the modern system, everything is a gamble (because that is the way the elites like it.) It's just that most speculations are gambles in favor of the state-bank system, at least implicitly. (If you buy bonds, you're implicitly betting that the dollars your bond will pay at maturity will be good.) These speculations have a bad reputation, since there are so many of them, they are often promoted deceptively, and their collapses are all over history. Gold and Bitcoin are two of the few ways you can gamble against the state-bank system -- they are a different beast altogether. They do have problems, but not the ones usually associated with most speculative bubbles.