I dont believe that Garrosh and his kor'kron thugs are enough to beat the entire Horde. Alone against one race, maybe, but not against them all.
The Darkspear and tauren's strength we'll see in 5.3 I guess, but they aren't the only ones in on the rebellion. Lor'themar Theron has been in contact with vol'jin, and plans to move agaisnt Garrosh too after he's done on the isle of thunder.
Blood Elves actually have a proper military (farstriders/rangers, blood knights, magisters, sunfury, spellbreakers, arcane golems, and now Sunreavers) and have a few big weapons too, like mana bombs and these new blood golems from 5.2. throw in whatever Sylvanas would offer (banshees, deathguards, dark rangers, etc) and Garrosh is in VERY big trouble. The kor'kron guard cant possibly stand against all that, especially with dissenters in the orcs too.
And now the Alliance is in the picture too.
I just dont get how Garrosh can win this. I know orcs and humans pull new populations out of their ass all the time, but even so it just doesnt fit. The Kor'kron might be enough to put a police state on one darkspear island, but even that is going to be destroyed in 5.3.