We think aiming for complete historical accuracy would also have been utterly boring for everyone except the dozen or so people who want a painstakingly accurately depicted Vexillationes Equites Sagitarii Seniores and a complete list of tribal factions and nations, all behaving like they were stuck on historical tramlines, with no choices for the player. Instead we create a start point that is a close approximation of an historical situation and then allow the player to alter history by their actions, thus probably creating a counter-factual outcome.
"When we grew up to school, there were certain teachers who would hurt the children any way they could...
By pouring their derision upon anything we did, exposing every weakness however carefully hid by the kids."

There's a Pink Floyd quote for every occasion.

Unless EB does something really incredible with BI, I'm not even going to buy it. If I want to play a historically accurate depiction of that era, I will play Maximus's Fall of Rome mod for MTW: a highly accurate depiction of the fall of the Roman empire, plus it lacks RTW's disadvantages.