fFirst, McCoy has incredible opportunity to work with the greatest QB in the history of the NFL and one of its greatest football minds, coach or player. Why would any offensive coordinator leave after only one year of this once-in-a-lifetime opportunity? Imagine how much more he can learn from Manning that will only make him that much better of a coach. Further, what an enjoyable experience it must be as a coach to have the greatest pupil in the history of the game. I have to imagine that to be extremely satisfying. It would do wonders for his resume and would help him basically pick where he wants to coach.
Second, a head coach does not succeed without a QB in this league. There are only 2 teams right now looking for a HC with semi-decent QB's - San Diego and Chicago. Further, there are no great QB prospects in this year's draft. The best QB available at a team with a HC vacancy is San Diego. Why would you stay in the same division with Peyton Manning knowing you are guaranteed two loses a season and will never win the division while Manning is still playing? He would have to rely on a wild card every year. So it just makes more sense to wait around another year or two, build his resume some more, and be able to pick a team with a really good or potentially good QB situation, possibly via a highly touted prospect coming out of college.
Third, he has some tremendous assets on the offense side of the ball, which will allow him to do some pretty special things over the next few years. Why not just enjoy that? Sometimes it is more important to just have fun and be happy in your work situation than to be solely focused on moving up the corporate latter.
In short, please stay, Mr. McCoy.