There's a difference between an average and a trend (downturn.) So, yes, NU has averaged 8-5 post-Osborne. But the majority of those wins were by 2 of 5 coaches, Solich and Pelini. If you look at it, both coaches trended downward during their tenures even though they had respectable W/L records.
Pelini won, on average, 70.2%. Riley won 50%, Frost has won 37.5%. Notice the trend? In fact, since Solich, the W/L ratio of a coach has gone up only once, Pelini won 20% more games than his predecessor. That means 4 out of 5 coaches have a worse W/L rate than the coach before him. (Granted, Solich, or anyone for that matter, was never going to match TO's success.) That is a downward trend. And when you compare Solich's W/L rate (75.6%) to Frost's (37.5%), beginning to end it is a downward trend of epic proportion.
Numbers are based on years since 1998, inclusive.