No problem with people saying that it would be great although unlikely. My issue was with people simply dismissing it. Maybe 600AD is too early, but as I mentioned earlier, much of the characters are already implemented (in ancestry in 1066). There is just the case of filling in the gaps and defining borders.
It's not just "filling in the gaps and defining borders". It's quite a lot of work, even making up fictional people and places, to ensure entire dynasties have all the characters in the right place in all the 900+ counties, duchies, kingdoms, and empires without any bugs, to make sure that all borders make sense without any bugs, to ensure that this, along with all the new mechanics, actually work as they intend to, without any bugs, and furthermore, and most importantly, to make sure that it's all balanced without any bugs (you thought the Fatimids were bad? What about an Abbasid Caliphate that's 5 times larger and covers half the map?). This is stuff that isn't for a DLC, even a large one - this is something more appropriate for a whole new game, or a standalone expansion at best. If these concerns are "dismissive", then, well, I'm more than happy to be dismissive and suggest this might be better suited to a different game.