The fact that an exchange will be targeted by hackers for online heist is hypothetically sound, just like 51% attack is. But you know, it's not worth it. And, if an assumption is made on the basis that an exchange is similiar to a traditional fiat bank, then, for business that exchange will have to come up with state of the art cyber security, or perhaps, in this context, crypto security which is alreday implemented in major coins, thanks to block chain. This article, in a nutshell, is a total time waste read.