If they were to take the game offline to perform the necessary changes then I can almost guarantee you they would be offline for a very long time, and I don't mean by just 1-2 days if they want to actually do anything meaningful. DDoS will always be around, there is no such thing as guaranteed protection against it -- Only thing it changes is the cost, resources and knowledge needed to take it offline again.

They are honestly better off looking at why people are doing a DDoS in the first place, which if I had to make a bet in part-in-play to do with the payment system changes they've made.