But... What if Garrosh is no ordinary Warrior? He could be training all this time... Got some levels... Maybe he's as powerful as Thrall, but in a Warrior-ish way!
I don't know how power increases in lore characters, but for me, Garrosh Hellscream could match Thrall in a duel. Maybe.
...and I don't want Thrall as Warchief again (misterious and supernatural reasons), so a death could be one way to take him out of the "contest"!
(Sorry for my english, not my native language)