That a huge structural fallacy is found, that will make it crumble to zero because it will immediately become unusable. The most possible option in this scenario is quantum computing reaching a status where Bitcoin’s cryptography can be broken in acceptable time, like some days.
 That would definitely destroy Bitcoin… but it must be said that there are countermeasures ready for this scenario, like of course different cryptography methods or empowering the actual method (which wouldn’t require toooooo much work). But anyway, Bitcoin’s price would suffer an enormous impact of distrust, until the new cryptography system is implemented.
In any case, if this would happen, I would buy Bitcoins after the price reaches the low, this is sure.