It seems to me - from bits and piece pieced together over the years plus my own observations being a student from 1997-2001 - that we were going to get bogged down without major changes. As you said, had Bo stuck around perhaps the defense could have made strides. But I also get the feeling that the same thing that made Nebraska great for a lot of years - assistants staying around - was starting to make the recruiting side harder. As some of those guys got older, the energy they had to put into recruiting was starting to wane. Plus the effects of the scholarship limitations were really starting to take hold. The Huskers couldn't stockpile talent like they could at one time and the playing field was starting to level significantly.
To be honest, at this point the last two years of Franks tenure look to me to be almost exactly like the first two years of Riley's. We finished .500 for the first time in quite awhile then got back to 9 wins the next year. But the nine wins were largely built on a weak schedule and we got drilled by the good teams we played. I'm not saying Frank would have crashed like Riley did but I don't think his last season was nearly as much of a rebound as many like to think.
And that's before you include anything away from the Stadium.