User blog comment:CoolJazzman/Why do you think Yamamoto wasn't giving a promoted to become a royal guard./@comment-8295743-20130417035134/@comment-8295743-20130417184034

Sen, I don't recall too much about the fight between Wonderweiss and the Old Man, but I am pretty sure his sword would have been useable again after Wonderweiss was killed. And yes, Yamamoto killed at least two of the Ritter with his Shikai, including the one who stole Senbonzakura Kageyoshi. (I think he actually killed three, before facing the Ywach impersonator).

The Old Man couldn't kill Aizen, I suppose, because Kubo Ichigo to do it. Yamamoto wasn't a main character. It's never explained in-story why Unohana, Ukitake, Kyouraku, Yamamoto or Urahara didn't release Bankai to kill Aizen. Even with the Hogyoku, I don't think anyone could survive a direct hit by Zanka no Tachi.

Oh, for the sake of being accurate, Kenpachi killed three Ritter before facing Ywach, and Byakuya killed an unnamed Quincy before having his Bankai stolen.