I actually like this. Milton is the type who will fold against challenging teams. Sam is the type that will scrap it out in a battle regardless who it is. In a battle, sam may have something like 2-3 int, but will also have 4-5 TD in the same game. Milton wont give you that. Sam has some of that vintage Kirk Cousins dawg in him. I like the move.
When sams back is against the wall he will put on the boxing gloves. He felt that way against Milton after the first 2 preseason games, and we seen what he did in the 3rd game (had some not so good plays, but he was letting that arm go to battle back into the game).
I am betting the cowboys wanted to see how Sam would perform under pressure, and they got their answer, not to mention he has shown that in real game action on other teams.
When it comes down to it, I want a QB that isnt afraid to put the ball in the air. Milton isnt safe, nor is he a chance taker.
Don't worry, it was the right move.