Minato would trump everybody in this whole series if he had his other arm!! You saw what he did to Juubito's TBB, this fight would be much more even if Minato had his arm Kishi did it for plot he wants Madara to have an epic ending!
U saw what happened when he had both. He was soloing left and right.. the manga would be boring and the war would lack meaning.
Besides he already solo'd the third war.