I get the Islam controversy point, but it doesn't make sense to me that Paradox can include Aztecs, but can't include anything else that actually happened.
The game really isn't built to simulate the early Middle Ages (and even less so the Migration Era), we would have to entirely rework the way holdings work, as well as technology. Not to mention actually researching a whole new start date, adding new cultures, religions and other related content.
Setting up the Sunset Invasion, a minor fantasy expansion with some light scripted content and some art, as an argument that anything can happen is faulty at best.
769 is already a stretch, we're not going to go earlier than that.