I have no doubts that Sylavanus, and whatever can be found in the Shadowlands can be defeated. However, Azeroth is going to have problems closer to home. Without the Lich King, the Scourge has no master and will run rampant across the world. And they outnumber the living 1000 to one.
There's no way Azeroth can defeat an lawless Scourge, only .01% of the scourge was lost in the war against the Lich King. The only way to defeat them, is to restore order. But how can that be done with the Lich King being presumably destroyed? Unless a way can be found to repair and reactivate the crown, a new ruler of the Scourge must be created? But who, or what should it be?