Quantum computers that could crack the cipher is a distant risk, as is a sun flare that would knock out the internet. They exist, but aren't likely at all. What's far more a threat is price volatility and the low utility of the coin. It's just not useful as a currency. The other side of this is that it is now reaching more of a consensus about being a store of value (despite the volatility). The biggest threat is that people just stop believing it will hold value in the future, which is the only reason it has value now.