I'd rather end up with Flynn or Tannehill than an almost 40 qb coming off of multiple neck surgeries who at best has 3 years left. Especially when you consider that atleast one of those three years will be spent rehabbing and getting back to 100%, and that at any moment Manning career could be ended by a hit that a younger, healthier qb could walk away from.
I realize Manning is/was an amazing qb and i would feel differently if he was either 5 years younger, or not coming off such an unpredictable injury. Its just not smart to give up everything to at best have a 2 year window that could be slammed shut at any moment because of Manning's health concerns, and after which you will be back to a mediocre(but not bad enough to get the #1 pick) team.