*spoilers ahead*
He is shown in the cinematic as an old orc, grieving for the loss of his son and wishing at the very long last to get his warrior's death. He gives up twice: refusing to kill Malfurion and tip the scales in the Horde's favor and allowing himself to be captured by the Alliance. He doesn't know how to lead, which ways are right in this new world, if honor still holds value. He wants to die and be relieved of all the wars, all the horrors he lived through. He is old, tired, broken, conflicted: he is not the Warchief the Horde needs — bold, daring, wise and cunning.
The best he could do is probably oversee training of new grunts, but lead the Horde? Engage in diplomacy, accept new races, conquer new lands for the Horde? The only leader he could be is a symbolic one imo. Disregarding the fact that he would be forever seen as a puppet installed by Anduin if he is ever appointed Warchief.