If you did split them up, you'd need to then write a melting-pot event which should have fully completed by about 50 years after game-start. Writing 5 or 6 distinct cultures (Mercian, East Anglian, Northumbrian, Wessexian, Kentish, and Anglish) with all the associated name-lists, retinues, unique cultural innovations, and so on, and a whole melting-pot event for Anglish which needs to fire reliably and have a reliable effect without leaving weird e.g. Kentish isolates floating around like CK2's sometimes to do, all for such a short period of gameplay is a big ask when you can just have "Anglish", one and done.