The Cowboys have a problem on their hands, and it's not necessarily good.
Here's the deal:
1. The Cowboys CAN'T pull Dak while the team is still winning. You affect chemistry and set up a scenario where the organization will be second-guessed and Romo will be second-guessed if we should lose anywhere on our journey toward the Super Bowl.
2. At some point, Romo has to get live reps if he's healthy. The longer you put that off, the more difficult it becomes to either put him in and/or give him a chance to knock the rust off.
3. As one poster has said, the playoffs is a different animal than the regular season. Dak is going to see some defenses he hasn't seen in the regular season. Assuming this overwhelms him, you don't want to have Tony Romo come in rusty trying to win for you - particularly when he's returning from an injury, albeit healthy at the time he is ready to play.
4. Unless there's MAJOR regression, you don't take Dak out. No, the Philly game would not have been the game to judge whether Dak is still worthy of the starting quarterback role. That would have done more to shatter his confidence than anything. You can't simply pull the hook on your quarterback just because he has a bad game. And Dak appears to be the future, so you definitely don't want to do that to him.
So what's the solution? Wrap up the No. 1 seed with three to two games left. Then you give Dak some playing time the first half, and you insert Romo in the second half of "meaningless" games. You evaluate both Dak and Romo to determine, for the former, if he, indeed, is mentally ready for the playoffs and, for the later, whether he has sufficiently shaken off the rust and has reestablished his own chemistry with the team.
It may be a good situation to have both Dak and Romo available, but it's definitely a delicate situation which could have disastrous consequences and potentially ruin the season.