Be it far from me to rain on speculation, but there are likely millions of times more good men than Knights of the Cross, and some of those good men are deeply involved in Heaven's work in ways KotC cannot do. See, e.g, the end of one of the short stories ("Warrior"?) where Uriel confirms to Harry that Michael didn't get a retirement: he got a promotion.
In "Skin Game", Michael didn't come out of retirement. When faced with a labor shortage, Upper Management told a member of lower management "retake your old position on the shop floor for a while".
