The azerite war machine failed, but the plague didn't until Jaina's appearance, whereas even Anduin starts asking his dad for help once Nathanos corners him until Gelbin and Alleria show up to save him. Sylvanas did intend to win the battle, but in the event she failed, she would ensure the Alliance get no more than a pyrrhic victory. I would have more understanding of Saurfang's character if his issue was a principled one about the plague used on their own troops and necromancy, less so if the plague is used only against the enemy in the case of a trap.