In Blizzard's game, the player always win, it doesn't matter how grim things are at the beginning, in the end, us the player will win. It has been like that since Diablo, and it has been Blizzard's way to give the players the ego boost and satisfaction so they keep playing the game. Afterall, it feels good to win.
In Diablo, Blizz's way to make the bad guy win is let shit happens between games, like how Prince Aiden ala Warrior went bonkers between Diablo 1 and 2, Rogue becomes Blood Raven etcetc.
In Warcraft, Blizz's solution to "Bad guy wins" is simple by making us the bad guy. Need to raze Lorderaon? We play as Arthas. Need to kill Cenarius? We play as Grommash. It doesn't matter if it's the good guy or the bad guy that wins the day, since in the end it's the player that wins.
None of these are possible in WoW since we can't play as the bad guy. While making us lose to the bad guy in the end is against the whole idea of Blizzard's formula of success, that is, the player wins.