I thought this statement from Bruce Schneier's cryptography book was interesting -
"It is important that the value of the data always remain less than the cost to break the security protecting it. "
You can see how relevant this is to the modern crypto-space.
Interestingly enough, the value of bitcoin itself is based on its security, and if someone were to break it, everyone would know its broken, thus rendering it worthless. An attacker with profit motive is no longer a threat, since there is no incentive to launch a '51% attack' on the blockchain. Such an attack would cost billions of dollars, and would yield no profit even after successfully being carried out. The world would then simply use another blockchain (some other coin).