The world could've been united another way also. If Naruto wasn't there at this very moment, the "joining together" would've been futile, seeing as they were already giving up hope, when Naruto got down.
It is the Kages that have united the shinobi again. Not Obito. It was the choice of the Kages, to make an alliance. So do not give the feat of the Kages to Obito, who had no intentions of uniting the world.
Let me be more specific.. it was actually mifune, who is responsible for this alliance's creation.