This unit has been a top defensive unit for years. It was under Lovie's tenure, soured under Babich, and was once again an elite defense under Marinelli. Rod is one of the most respected defensive coaches in the game. I know that you hate the Cover 2 but last I saw the 3-4 was torched by the Niners (vs. Packers). The fact of the matter is the Bears defense has worked and it has worked incredibly well. Not only that, I've seen young members of the defensive line take steps forward under Marinelli (Wooten, Melton, etc). You hear what ex players have to say about Rod and that says a lot. Not only that, but he's a leader and a voice that our players relate to and our comfortable with.
They might find a great new guy to be the DC, but when you have one of the top units in the league (and this past year the defense played at an absolutely elite level) you damn well aught to be concerned when you lose the coaches which led that unit. Especially when you have an aging unit with a lot of talent still on it. Makes zero sense to change schemes at this point in time. Not when you were a 10 win team that underachived. That is a team with superbowl potential and it makes zero sense to risk what could now be an overhaul to the defensive side of the ball.
I also think it is very clear that trestman doesn't have the relationships with defensive guys (now I do think if I were a top defensive assistant I'd want this job cause of the talent) but this is a major major blow.