You are simply pointing out how it is and not how it could be. Nothing about Arthas or Deathwing goes against the posibility of good story telling. You say, that should go for books for good stories? I say, that i demand more of the medias i consume then just let it be childrens fodder. Besides, one can still make a good story despite having it be kid friendly.
Every once in a while we get a good story in WoW - Wrathion in Cata-MoP, Farondis in Legion and the Wildhammer storyline in Cata, so it is not like Blizzard is incapeable to make satisfying stories... Its just that they apperantly are so incapeable to understand when their stories hit right and when they miss, that they are putting things up for random. Like, i am not a second in doubt, that we will get 1 good story in BFA, but i can already tell, that we will highly likely not get more than just that one out of chance.