I don't really have any issue with what Diaco said, btw.
Reality is, at the end of the day he's not allowed to say things like, "Hey, we'd be an elite defense if we didn't have a borderline top 30 roster talent-wise." At the same time, every coach's approach to teaching from scratch is different, and some have better immediate success than others. I'd have zero issue whatsoever with Diaco being around next year.
Our 2008 defense was 81st in the nation in scoring defense. Our 2009 defense was #1. Pelini benefitted from a roster loaded with NFL talent, make no mistake, but it still took a year of arguably bad defense for him to really turn it around.