I also think Barry knew he had subpar safeties and played them extra deep. He was scared about getting beat over the top for a big play. Problem was he played them deeper than he needed to IMO. Also, losing Rasul, and Stokes/Jaire for most of the year made him nervous at corner, the *allentines did ok in zone but when you play a lot of Cover 2 or Cover 4 (or other MOFO variants) you're leaving the short-to-intermediate areas open, and Campbell has lost his speed and Quay isn't the most instinctive in coverage. Rushing 4, dropping 4 deep and leaving 3 to cover the rest is tough and lets offenses dog walk you down the field for 8-10 play drives.
I'm not completely sold on Hafley and wanting to play a lot of Cover 1 - it's going to expose the defense to a lot more explosive pass plays. GB does have pass rushers, so that's the saving grace there. One thing about Hafley based on interviews and all the background pieces coming out is that unlike Barry he seems to be adaptable with his scheme and play calling. Barry was very much attached to a specific scheme and loathe to deviate from it. Hafley has worked in a few different schemes as well which should help him adjust, but I'm taking a wait and see approach with him as DC.
One thing is for sure: he'll need a safety who can actually play MOFC deep Cover-1. The ones who can do it well aren't common. The really good ones are HOF types like Ed Reed, Earl Thomas, etc., which makes me wonder if it's viable to play that a lot (maybe they go Cover-3, but Jordan Love just absolutely roasted Dan Quinn's Cover-3 in Dallas). Hafley is also going to need speed (and instincts) at ILB; Quay and McDuffie aren't enough. Even if they only play 2 ILBs at a time they need a 3rd for depth - Quay and McDuffie missed time with injuries last year, and competition-wise neither player is irreplaceable.