The problem is AA guns and SAMs are specialized. ACs have never had ultra long-range attacks other than sniper rifles/cannons and railguns, and the missiles have never been very good at distanced combat. Something flying at, say, 15,000 feet would be immune to counterattack from an AC. Granted, this is taking realism into account, which is fairly pointless. If we do see jets again, they'll most likely be the way they were in AC3/SL, flying at about the speed of a low-power overboost and making predictable gun/missile runs. Although it would be nice if they were like the jets in AC1, those had a nasty tendency to fly past you at high speeds and spam their high knockback machine gun.