Why not? They proved to be the best two QBs on the team. I just don't get the thinking that somehow Moore is going to all of a sudden save the day if he has to play. Rush proved he is much more likely to give the team a chance to win. He is the one that should play if Dak goes down.
I don't believe the coaches grade players just on preseason game performance, for one. Plus, there's a world of difference between preparing for second or third stingers in a preseason game where nobody's game planning and you're looking at relatively vanilla defenses and going in live in a big game with no snaps in preparation against Steve Spagnulo's blitz-party. They're not going to want to risk Rush's development, and they are going to want a veteran presence in the QB room for both Dak and Cooper.
And I know nobody wants to hear it, and that just saying what I think the rationale is makes me the poster boy for Kellen Moore when the reality is I think he's a bottom-third NFL backup. But that doesn't change reality. This writing was on the wall all along, and the team said as much coming out of camp. Cutting Moore was only part of the puzzle of getting the roster together. He's going to be the backup until they think Rush is actually ready to start games whether fans like it or not. That's the hurdle: whether or not the team thinks he's ready to start an NFL game. Not whether or not he's better than Kellen moore.