What you don't seem to understand is that what damns the Horde is that Garrosh could get into power, do the things he did, and still maintain enough political power to strong arm anyone. You know that shit Vol'Jin talked to Garrosh ages ago, about how everyone would come to despise him and his "power" would become meaningless? That didn't happen. Garrosh was the one who turned on the others. Their "rebellion" was nothing but self defense.
There's a version of this story where the Horde aren't the villains. It's a version where Garrosh orders a nuke dropped on Theramore and his soldiers refuse. Or a version where Garrosh nukes Theramore, and a horrified Thrall challenges him for leadership the next day, beats the shit out of him, and executes him for war crimes. Or a version where Garrosh orders a bunch of Theramore refugees tortured and everyone in the room just plain doesn't do it. Not the version we actually got.
Garrosh may be a strong warrior, but the only thing that made him powerful was that fact that people inside the Horde were willing to listen to him and do what he said. Without that he'd just be some big strong monster with no power beyond the reach of his axe.
If only a few people inside the Horde were willing to obey him, he'd have ended up isolated, a poser "Warchief" like Kargath Bladefist or Rend Blackhand and their little false Hordes. He'd have ended up the joke Vol'jin predicted. Instead he was able to get enough people to listen to him that everyone else had to whimper and be whipped so badly that they only finally rebelled when Garrosh came directly after them and they had the Alliance to help.
A king no one will obey is just a guy yelling orders in an empty room. The Horde, as a whole, always had the power to leave Garrosh yelling orders to an empty room, but too many of them chose to listen to him instead.