It's probably a similar matter to things like a cutscene skip feature, where the primary issue is just that there's a very large number of things to go back and alter a feature for. Which is understandable, but it's honestly something you could gradually work towards, similar to how they've been gradually adding markers for various things on the maps in game. They could just do it in waves, e.g. first the new versions of battlefields, then old AU content, then old Promathia content, then old zilart content, etc.