A coach goes 7-7, then revamps the defensive staff and the team has a record of 10-3 the next year, albeit with a kind schedule. Ranked in the top 25 after the season. I'm sure the fear was of having a true losing season, but NU did that with Callahan the next year. Frank would have been gone the next year had he gone 5-7 and missed the bowl. I suspect the fear for Stevie P was that with Pelini in the fold as D coordinator, Frank would have posted 8-9 wins as long as Bo was in the mix, thus putting NU in a kind of purgatory. I think there were 8-9 wins on the schedule in 2004 under a Solich/Pelini staff.
The fact is that we think we know what would have happened if Solich remained, but we really don't. The general operating procedure since TO left has been to not be happy with the coaching, and thinking that if we could just be rid of that coach, some magic coach was was waiting in the wings to take NU to new heights. The result is that we would all be incredibly happy if the team won the same number of games that Frank and Pelini won every year but one.
A 7-7 record is a reality that even great programs had experienced much more recently than NU. So no, that wasn't a downfall. It was a sign for a corrective, which NU made. Was Solich an inferior head coach to TO. Yes. Was it wise to fire him after a 9-3 regular season and a top 20 final ranking. No, unless you wish to signal to the applicant pool that insane expectations are expected.