Recently we saw bithumb getting hacked, following the predecessors case. Sholud all exchanges be decentralised to aboid hacking?
Not all, but we should see more decentralized exchange and its development. The currently available decentralized exchanges are far inferior to the centralized one: the volume is low, can't trade between two different blockchains, and the most frustrating one is the latency issue. Even in a slow-moving market, it takes several minutes for an order to go through and gets executed. Not to mention that there's still a way to "hack" the supposedly hack-proof exchange by phishing, DNS hijacking, etc. like what happened to EtherDelta last year.
Hopefully, there'll be many improvements, especially in operation and user experience side, in decentralized exchange in the coming years as many projects are working on the field.