There, thats the fact of the matter. For the last several years alliance players have been whining about how they feel left out or how the horde has beaten them at everything and it made them feel underappreciated.
So to give alliance players something they decided 'hey, you get to kill the hordes current warchief'.
It really breaks down to being that simple, despite people trying to overcomp for reasons why, because alliance have been whining to endlessly about it, the blizzard devs decided what better means to make alliance happy then to fight and win against the current horde.
Now as a horde player, I'm glad Garrosh is going, he needs to go, he's made the horde into a sick parody of what he once use to be. However, this the thing I find baffling in this whole debate.
Alliance players have been whining for a while now about feeling like the underdogs (which the horde use to be before), and yet there has been no cases of the horde absolutely beating the alliance in the last ten years of the games story. There has been turf wars, there has been throwing of things back and forth, yet there has not been any absolute victories, all blizzard did in cata was balance the zones to give a balance percentage to the horde where the alliance had more before them.
So, what this breaks down to is the reason Garrosh is going to die at the ends of mists is to keep alliance players happy. I say this not because I'm unhappy about Garrosh dying, he needs to, I'm just making it clear all this was down to insipid complaining from the alliance players for not feeling like they are the main focus. That they need to score an absolute victory against the horde to make them feel like they are winners.