Say they do fire him. What then?
Well, you have only 2 choices:
1. Promote one of the other defensive coaches to DC.
2. Hire someone from outside the organization to be DC.
In the first case, if those coaches were DC-caliber, they'd not be on this team, with Jerry running the show. So not likely they'd turn the defense around to any great degree. Making the defense a bit better isn't going to get this team to the SB.
In the second, so a new coach is going to come in and the defense is magically going to improve a lot? With this roster? You don't have a bonafide top pass rusher, one of your starting CBs is out for at least 3 more games, the DTs are mediocre, your best slot corner you let get away, your safeties are average at best.
Pointless to fire him at this point, best to work at improving the roster first, then see if he can do well.
After all, have you looked at the playoff standings currently? Cowboys occupy the 11th spot. With the resurgence of Seattle, etc. it's probably already past the point where this team will make the playoffs, and even if by some miracle they did, ain't winning it all, you're just dropping down in the draft, with the problems I noted above, you need all the high draft picks you can get...