We didn't really beat the lich king, though. He just sort of sat there not doing anything while we slowly made our way to him, then he just died. Sort of like deathwing.
If either of those two had actually been trying to win, then first the entire world would turn undead (he wouldn't even have to fight; he'd just kil all the trolls with diseases, or alternatively kill all their food if they were somehow immune to it), then deathwing would come along and blow it all up, or whatever his ultimate plan was, and that'd be that.
Makes for a boring narrative, though, I guess.