So why not simply make a post 395AD mod?
1) Mods lack the level of polish which the vanilla game has - CK2 is worked on full-time by paid staff, whereas mods are hobby projects which get juggled alongside other commitments. Not only that but vanilla has the entire community to search for bugs and help with accuracy whereas mods only have their far smaller player bases.
2) Continuing on from my point about polish, mentioned above, the difference in the time spent on mods vs dlc also gives less scope for details.
While I agree or am neutral about your other statements afterwards, I have to disagree somewhat with these points, particularly as a modder-in-training, and I feel it is necessary to defend my fellow modders here. Yes, it is true we are not paid and doing this only as a hobby, but this does not necessarily mean we can't work with bugs and accuracy - if anything, a good number of mods are even
better at bugs and accuracy than the vanilla game is. Modders have much more time to think about and apply historical accuracy - look at mods like SWMH or Lords and Lands, for instance, that meticulously rework the map and dynasties of Europe to fit medieval reality better, without trying to make things "balanced" for the player; as for bugs and other sort of issues, mods like Project Balance have been fixing balance issues and the like with the vanilla game - such as the infamous overpowered Muslims/Fatimids issue that PI only got around to somewhat fixing in 1.08 (but not really) - way before PI got to it. Another example of bug fixing would be the Elder Scroll's unofficial patches (I know it's not CKII, but it demonstrates my point admirably) - these modders managed to fix way more bugs and glitches in the Elder Scrolls game than the devs could ever hope for. Precisely because mods are hobby projects where modders aren't paid and do it in their free time
because we want to, we have much more leeway when it comes to accuracy, bug and issue fixing, and so on. Even though PI has the entire community to help them with bugs and accuracy - will they really take in
everything that is reported, requested, and so on? No, especially considering that 1) they have, oftentimes, worst time constraints than modders, and 2) they simply can't add anything thus because they have to work on other aspects of, well, their job. Do other things, see. Modders, if they're dedicated, can focus on a few things at once.
Certainly we cannot market ourselves as well as the vanilla game, but this doesn't mean the mods are of any less quality. I do not claim that the devs are useless, or that modders are superior or even equal - otherwise we wouldn't have CKII in the first place, of course. But remember the devs can't make everything that everyone asks for. Modders can't either, but they can come a bit closer to that.