IMHO, he couldn't carry Letterman's jock strap. The transition should occur in 8 months (end of his current show's run) to a year, I've heard.
The further fragmentation of network TV audiences to other cable choices will dilute and scatter Letteman's audience to the 4 winds.
FWIW, sounding like the old codger that I am, I go back to Steve Allen's show and Jack Paar show. Letterman wasn't quite up to the level of them and certainly not Johnny C either... but he was quick witted, funny with his off-the-cuff banter and self-effacing manners. Furthermore, he was a damned good interviewer, too. More than 10-15 yrs ago, when he was in the old format, he was far more interesting and creative.