^ You're ignoring the context of that statement though. Isshin asked if Aizen had hit his limit as a taunt, and Aizen replied that he had hit his limit as a Shinigami to indicate that he was about to become something else. It was nothing but clever double talk, along the lines of "you already died 100 years ago" and "reason exists for those who can't go on without it". Not everything is meant to be taken literally.