In the end Bitcoin is so similar to Gold. It is a means of storing wealth and is valued on a shared belief of finite quantities and that it is an effective/secure means of storing wealth. It is hard for me to call a shared belief a bubble, because there is a clear understanding I think now that there is no underlying fundamental asset value.
A small number of people think that now because Bitcoin is in a huge run up. If bitcoin crashes 90%, it suddenly stops being a "secure means of storing wealth" and, instead, is a means of throwing away your wealth.
If the market psychology changes, bitcoin becomes worthless.
Gold has the same problem, but has a huge historical record of actually being somewhat stable. And there is a price floor for practice demand of gold, though I'm not sure what that is.