I don't think you can call Bo "average" or "mediocre". I think you can call him good with a more-than-likely realized ceiling. 7 years is a long time to show a good chance at taking the next step, or to at least get lucky, and also a long time to slip up. You can look at his tenure from all sorts of different positive and negative perspectives, but at the end of the day, he wasn't really any further or any closer to championship level football than when he got here, which is wild to think about.
I really don't understand how you make the jump to the bolded. This was a guy in his first 7 years as an HC. If anything he established what appeared to be his floor (or at least average level of performance). We know that experience tends to lead to better results after more time from our very own Husker history.
Football, especially in college, is never as linear as what you and others who were looking for "progress" or "a next step" would hope for in a 7 year span. And winning a conference championship is NEVER about luck.
I also disagree that he wasn't closer to winning a championship when he was fired than when he got here. I think the roster this year is far better balanced and in shape than it was in '08 (yes, yes, I know Suh was on that roster). I also think he'd gained valuable experience. The one negative in my mind is that he'd always suffered too much assistant turnover. But other than that, I think NU would have had a break out year this season against this schedule had he stayed. As it is, this was one of the weakest schedules in years, and if NU had put beaten even just 4 of the teams that it lost to, that schedule strength would have dropped further.
This year was a definite missed opportunity.