There’s a dramatic reason why the battle for Winterfell was written as it was. It allows the viewers to believe that everything depends on the forces initially arrayed on the battlefield; it allows us to believe that hope is fading for Jon and his men as the circle of spearmen close around them; it allows us to fear that the worst is about to happen. I felt that; didn’t you? And then I felt a great sense of relief when we heard the sound of a horn and Ramsay looked off to the side and we saw Littlefinger and Sansa and an entire fresh force of cavalry entering the fray. This is what “victory snatched from the jaws of defeat” means. That we heard nothing of her plan ahead of time is a problem, I agree, but it’s possible to imagine at least a partial explanation for it, and the way it played out tells us as well as Jon that Sansa’s contributions are vital, and most of all this ploy on the part of the writers gave us a great emotional payoff.