Hackers mainly focus on exchanges, so try not to store their coins on the exchange, so that they can be relatively safe against these hacker attacks.
They target anything that is going to be a treasure chest of money and coins full. Not something that is going to be a farmers' piggybank, obviously. Exchanges have a lot of coins in store for the customers who are using the exchange and the funds of the exchange. Now it is not possible to always withdraw all you balances because trading goes on with new orders being placed and new more deposits coming in. People who do arbitrage again has to have funds ready to deploy.
For the long term though one should try to keep a decent amount withdrawn away to local wallets and not keep them on exchanges.