Well, it's a conundrum, actually.
In my opinion, NU should never fire a coach who's winning 9+ a season, because it's unlikely that a new coach has a better chance of bettering that result than the existing coach has of "figuring it out" and bettering it.
However, with Riley, he'd be really old at that point, so the chances of him figuring it out are diminished.
I'll add one other thought, which I know isn't popular, and maybe I hold onto it because I really don't have my ego wrapped up in Husker football: .700 and a clean program is completely acceptable to me, and though I'd love for kids to win championships at Nebraska, I think that's a valuable and meaningful experience for players to be part of a consistently solid program.
I just won't get on board with firing a .700+ coach in hopes of getting an .800+ coach when I realize that downside risk for these players is so great.
If Nebraska had kept Pelini and he never won a conference title and never won anything more than 9 games for the next 25 years, but maintained graduation rates and low off-the-field problems, you would have been completely OK with that? Does that also mean you would've never issued a gripe or concern about this program getting over the hump?
I find that hard to believe.
Regardless, even if that is true, I think most people understand there are risks associated with firing a .700 coach in favor of trying to find one who is better. The problem is that this university, this fan base, the economic machine that is Nebraska football, wasn't built on .700 win seasons. It was built on winning conference titles and championships. That's what built the brand and the fan base, and what drives literally tens of millions of dollars into the state for every year.
If Bo Pelini had stayed and never won anything of substance, attendance would've dropped, profits would've dropped and the fan base would've continued to stop caring.
I get what you're saying but it just isn't realistic.