For two expansions the Alliance, including its playerbase, runs roughshod over the Horde, and every time the Horde tries to fight back they fail miserably. It isn't until one day Alliance players decide that they want a different king that the Horde is allowed to start winning, and only against an NPC faction. After Varian is killed the Alliance gets to keep all its holdings and the Horde is okay with it.
When can the Horde expect to run roughshod over the Alliance for two expansions then? Because it sure as hell wasn't "Oh crap Alliance forces invading every single one of our starting zones and all of our offensives are failing horribly." Cataclysm.
I was working from the original post, which proposed that the war continue after MoP, and that Thrall be responsible for it. I simply added the suggestion that he be given an actual reason to do so, rather than just "lol kill the Alliance".
If you've another idea which makes Thrall responsible for the war continuing beyond MoP and provides a clear reason for doing so for both sides, share it with the class.