Tulip bulbs.
Quite apart from the bizarre investor psychology which is fueling this price rise, there are two main issues in my view.
Firstly, blockchain is clever technology but the lack of a centralised register makes it impossible to forensically replicate ownership records if the system ever crashes. Of course, the glib answer is that a distributed register can't crash - but I don't trust that the access enabling technology will hold up if all the investors head for the exit together.
So what might cause a stampede for the exit?
In a word, regulation. I suspect that the emergence of crypto currencies is so disrupting for State structures globally and their taxation, oversight and law enforcement agencies that there will come a point where Governments worldwide have no option but to regulate/intervene. How much of the growth of Bitcoin is based on criminals engaging in money-laundering and Russian agencies sanction-busting? What would be an acceptable percentage?