Izuna would add very little if anything at all to the story. Obito is a fallen hero who inspired the principles Kakashi lives by and taught to his students. He succumbed to darkness when he was at his lowest and embarked on a villainous path that sought to drag the entire world down with him so he could start anew. He is both the same and the antithesis of Naruto, and that adds weight to his words and actions more so than any other random Uchiha could.