I knew this argument would pop up, but it is very flawed thinking. You are assuming that if Elliott downed the ball, that 1) the Cowboys could still score a touchdown when they wanted to time-wise, and 2) the Steelers would have no time left to come back.
#1 is the biggest part of the flaw. Let's say Elliott downs the ball instead of scoring. The Cowboys then run three times and failed to score a touchdown. You can assume 50-55 seconds at most have run off the clock by the time they kick a field goal because I believe the Steelers had 2 timeouts left.
So, now the score is Cowboys 26, Steelers 24 .. and that's assuming the kick is not blocked, placement is not bobbled/fumbled, etc. Now the Steelers get the ball back and a field goal wins the game. Playing scared in a situation like that is what the Cowboys used to do and how they have lost so many games in the 4th quarter.
The bottom line is that unless you have the lead and you can run the clock out, you take the points, especially a touchdown. You do not assume you can score a touchdown on the next 1-3 plays nor do you assume that you will make a field goal simply because it is short. You take the points and trust that your team can stop them with the remaining time, and in this case, it required a 75 yard drive to take the lead by the Steelers. If you want to blame someone, blame the defense for giving up a quick and easy touchdown.
I can only imagine what would have happened had Elliott downed the ball on the 1 yard line and then either the Cowboys fumbled on one of the following plays or the field goal was blocked. Everyone would be calling Elliott an idiot and saying Garrett is the worst coach ever.