Yeah, I agree that the roster issues will take time. As the coaches acknowledged, we simply added an army of freshman LBs but it's not necessarily ideal. DE recruiting going back several years now, it's been an issue and it doesn't look like it's being addressed effectively currently. If players step up it will negate some of this, but if not...it'll be a few years.
Granted, we don't know if Riley is going to leave a well recruited, well managed roster in a few years either, or if he'll fail and it will be littered with its own deficiencies. Then again, Callahan's roster management was criticized for reasons like over-reliance on JUCOs, but he had (or had good reason to, at least) to go that route with what he was given. So the effects can last for years.
However, disagree that Pelini is not a good defensive coach. His terrible results here had a lot to do with how he was able to hold up as a HC in the spotlight. When we fire a coach we tend to get into hyperbole about his ability and it's often unwarranted.