You've said he's a bad DC never mind a bad coach. I've said he's a very good DC and an average coach. I think he's a Sean Payton, who's very overrated in mind, in that he needs a coordinator on the other side to run things for him. He'll never fix an offense. He'll never be a team builder.
Here's a list of coaches from last season. The top section clearly better than Rex. The second section is probably better but the all overrated team IMO with O'Brien thrown in there because I think he'll be good once he gets his QB situation worked out. I also put Ariens in there because he looks to be rising given Arizona and Indy. The bottom list I would take Rex over. You can debate a few of the names in all the levels but I think it mostly stands. If we did DCs which you feel he's even awful at that I'd put him in that elite level.
Belichick, Kelly, Harbaugh, Harbaugh, McCarthy, Coughlin, Carol,
Tomlin, Payton, Fox, O'Brien, Reid, Fisher, Ariens
Mike Smith, Lovie Smith, Caldwell, Marrone, Rivera, Lewis, Pettine, Garrett, Bradley, Philbin, Pagano, Weisenhunt, Zimmer, Gruden. McCoy, Sparano