Your post has had me thinking a lot mate. The bit about defensive coach being needed.
I have never been impressed by Bailey Wright. I think he is vulnerable and creates vulnerabilities in his defensive partners. I have had those doubts since he came. Johnson had him as skipper at Bristol and seems to have backed him here too. It has made me wonder if Johnson is as good defensively as he is through the other two thirds. I have seen a few coaches over the years who cant do the defensive part. It is the easiest bit to coach, but for some it isnt what they get. In youth football you see it way too much where defensive coaching is alien. Anyway I was thinking about Johnson's time here and we have probably never been secure at the back, but have been a joy (a lot of the time) when going forward. Last season he probably got lucky with Sanderson and O'Nien. I said at the time O'Nien may well be a CB for the future but he is back in midfield. Our most organised defence was when MacLaughlin played because he was just a pure defender, but he was let go.
All this makes me worry Johnson has a blindspot re defenders. Hope I am wrong. Whether a defensive coach is right I dont know. I have always thought you coach a team not a subset, keepers apart. I wonder if we have anyone on the staff who wants to really hammer the organisation into the back 4 or 6? Heavy defeats are often signs of either mental fragility in players, or weakness in defensive routine. I dont know but you have got me thinking mate.