That defense was never going to stop Aaron Rodgers, Russell Wilson, or Tom Brady in the 4th quarter. Bad defenses are exposed even more in games that matter.
I agree, but with a ball-control offense, the plan is for the Cowboys to be protecting a lead in the fourth quarter. That defense did stop Russell Wilson in Seattle a couple years ago, and maybe it would have stopped Rodgers too, had the Dez catch not been overturned. Now we'll never know how that game would have turned out if Dallas took the lead; because the Packers would have been playing it differently if they were behind and had to score.
I understand the whole "defense wins championships" mantra. But plenty of teams with mediocre defenses have won Super Bowls if the offense was good enough. The 1993 Cowboys, for example, had a very average defense compared to the 1992 version.