Some exchanges have been already hacked so I wouldn't be surprised if one of the biggest exchanges was hacked. Exchange called btc-e was hacked recently, wasn't it? Blockchain technology is secure and the biggest threat is quantum computing which will compromise all available crypto algorithms. However, we are still far from building a cheap and small quantum computer so I wouldn't worry about it for the next 50 years. China has already banned exchanging Bitcoin but it is also thinking about lifting the ban.