10 is the limit for Exaltation because that's the highest anyone can have while containing their Essence in a single soul. As powerful as they were there's no known record of the Exalted ever creating new ones. Only the Primordials have greater Essence, fighting these titanic beings was an act of heroism eve for fully developed Solars and their allies. maybe some truly mad Solars, driven mad by megalomania and the ability to do anything they desired attempted to raise themselves to the level of the world's creators, elevating them above even the Unconquered Sun by experimenting on themselves. If such things took place there's no known evidence of them succeeding. But that might not mean they didn't try. Their abilities were exceeded only by their own ego.
Perhaps this was the purpose Amarec had when creating R'yuk, to create another soul for himself and thus be able to increase his Essence even further. Of course according to existing logic on the nature of the Primordials he would have to create ten more, which maybe he planned to do. His notes on his experiment were extremely vague about his actual purpose so any thought on the subject is a matter of speculation. A similarity with Horcruxes enters here about keeping parts of yourself in objects. Maybe on his journey to Malfeas he really did learn the secret of the Primordial's existence, or at least the relationship of demon princes to their master and then discovered a way of reproducing it. If this was the case, since he'd already begun he was on the path to becoming a being of such transcendent power. If so then the Sidereals were justified in assassinating him before his plan could be completed.
