You are 100% right in saying that you must not fixate on the weapon, but you can't ignore it either. You must at least try to control the arm with the weapon while you are hitting. A lot of knife defence is taught at range where you can move out of the way but in reality, once the attacker commits to the attack you will be inside grappling range. In close it's knees and elbows, you won't get a clean strike. Sure it's going to be simple (gross motor), but it's not safe and it's not easy. If the knife arm is not controlled at that point you are in real trouble.