Personally I have to agree here. While I don't see this ever happening unless Calahan fears for his job, Cosgrove is not doing his job as Defensive Coordinator, and needs to be let go.
Look, I don't care how much time you give a coach... if he can't teach his linebackers the basics like how to tackle and how to stay in your lane, that's bad coaching. Yes, Bo did a hell of a lot better with the defense... that's a fact. Back in the Solich days the roles were reversed; our defense was solid (with worse recruits) but our offense couldn't get the job done. Now our offense is showing promise, but our defense plays downright horribly, even with higher star recruits.
Face it, you can get the best 4 and 5 star recruits you want... and if you don't teach them well, and don't call the right plays, and don't adjust to the opponent's schemes, then your defense will suck no matter what. USC giving up and letting us score a few touchdowns in the 4th quarter doesn't excuse 35 unanswered points. They scored more points on our field than they did against Idaho at home. This should be embarrassing, it should be brought up, and coaches should take responsibility for it.