I think it's fair to argue that the Steelers offense should have done more earlier in the game and never put the Steelers in that position at the end of the game.
However, late in the 4th quarter when it matters the most, the Steelers offense scored a go ahead TD and took 5:30 min off the clock on that drive. So, they gave the team a lead when it mattered and the drive burned clock, giving the defense a rest.
If you want to be a great defense, you have to close that game out and get the stop on Dallas there to end the game. The game was on the line, it was in their hands, and they didn't make the stop. They didn't even force a 4th down until the 4th and goal and that was only because of the crazy dive, effort, luck on the 2nd down with the forced fumble.