Now that I think about it...

Why not keep Peyton Manning? Rumors of his health, or even retirement have surely sabotaged his value. And then, there is that number one draft pick for a "surefire" prospect in Andrew Luck. That's a pick that, no matter what, still has value. The Colts are quite obviously more than a quarterback short. The idea of getting rid of Manning, in my book, isn't all that palatable if the Colts get nothing in return.
So, again, if the Colts can, perhaps, grab multiple first round picks, grab a player like Justin Blackmon to replace the likely to depart Dallas Clark and Reggie Wayne, and a linebacker like Dont'a Hightower, because the defense sucks, and take your chances with Manning's health next year, while drafting a quarterback in the later rounds and adding, say, Colt McCoy from the Browns with the same trade of the #1 pick.
Sure, it's a risk. Of course it's a risk, but so is letting a hall of fame quarterback walk for absolutely nothing.