I agree with all of this, but can't help wonder how much the talent on the field was really the issue. To me, this D is built on pressure from the DL, CB's who can cover (to some degree, they don't have to be world beaters) and safeties that are a true last line of defense and ensure nothing gets behind them.
Well, we didn't have a Willis/Norton/McIntosh type, the safety play was borderline bad -- probably the worst it's been under Diaz (especially when Bolden stopped playing well), the CB play was awful and the LB play was the worst we've seen in years.
As of now, the DT's are what they are, but Simpson can hopefully coach them up better. The safeties will get better in time as the room is loaded. The CB's and LB's are still a huge question mark.
Point being, if Manny had just been DC in addition to HC this year, would our D have actually played much better? I always viewed coming out flat-footed as being on Diaz, so it's not really germane to this.