Ok the main thing about this is that I'm positing that Bo wasn't ready to be a head coach after 2003. While I won't say that Callahan was a great hire (he did have a couple of productive years, but overall his stay here was a failure of the highest degree), I don't know how much better Bo would've been if we had just kept him here.
Say what you will, but he earned a whole lot of experience in the years between Nebraska. He was on National Championship contender football programs for all of his years, he probably learned exactly how to run a program like Nebraska from Stoops, and he learned how to win at high levels at both but mostly LSU where he won a National Championship.
So while we have no clue how good we would've been if we would've just kept him, I don't think anyone can disagree that coaching at top programs like Oklahoma and LSU didn't prepare him to bring Nebraska back to prominence.
(I'm pretty sure that this probably would've been better posted when he was hired or anytime before this, but I just found this message board so I didn't have a chance)