Why does Yishmael exist? To force, through war if needed, Aharon to merge with Moshe. Why does Edom exist? To force, through war if needed, Moshiach ben Yosef to merge with Moshiach ben David. By "merge" we mean self-cancellation in view of the Truth, the Tikkun of Moshiach.
And if one would argue that this would leave us with eight Sephirot, let him count Keter double, and let him count Da'at. Thus, in the world after the Tikkun of Moshiach, Atik Yamim is on top, above Arich Anpin, above Da'at, above Yisrael/Tiferet, above Moshe, above Moshiach ben David. These six sephirot are accompanied by Chochmah and Binah, and by Chesed and Gevurah/Din.
