Let's see now, Ichigo beat Kenpachi before learning Bankai. After learning Bankai, he beat Byakuya. Aizen stopped Ichigo's attack with one finger before obtaining the Hogyoku. Now, If Kenpachi couldn't beat Ichigo, and Ichigo couldn't beat Aizen back then. Aizen was only later defeated through the Hogyoku's rejection and then being sealed with a kido invented by Urahara, this was only made possible by the fact that he was made weaker by Ichigo using The Final Getsugatensho. Now, if it took all that to beat Aizen after he obtained the Hogyoku and even before obtaining it, Ichigo couldn't even harm him, then I highly doubt that Kenpachi would be able to beat Aizen.