Thornton and Allen are gone if they have any chance of being drafted. Frank Jackson is legit. So is a sophomore Kennard. Then you've still got Matt Jones, who would be a senior. If Tatum and Giles play the 3 and 4, and K goes with Jeter or Obi at the 5, how does he play five legit guards for two positions without a transfer or some dissention? UK pulled it off with four guards for two spots. Imagine if they had one more guy vying for minutes in the backcourt last season. Chances are, someone would have left the program. That's what Duke would be facing in your scenario, and historically K's not immune to transfers. He doesn't like to play many guys, and I doubt very much that he'd entertain the idea of a platoon next year - not when he has two of the top 5 picks on his roster.
I think at least one of either Allen or Thornton bolt after this year. Jackson's not coming to Duke to play 10 minutes/game. Neither is Kennard.