Here is my take on it. If I hack the blockchain, and do something nefarious (double spends, steal coins, or whatever), then the security flaw will be noticed pretty quickly. So what happens? I think that bitcoins quickly become worthless. So there is no point to hack it, because there is no financial incentive. This is why I think hackers are more likely to focus on hacking exchanges or individual's wallets in stead of trying to hack the blockchain itself.