Re: SURPRISED; WHY is 3K forums so DEAD relative to other TW titles?
I think the issue originates from a combination of all these factors you mention. Firstly, because of Steam and the rise of social media, the forum landscape has changed dramatically. Younger players prefer a different, more fluid format, where replies are quick and easy, while modding is now largely dominated by smaller modifications and not by large overhauls, like what happened in Medieval II or Rome I. In what concerns Three Kingdoms specifically, the crux of the matter is that mods are not only few, but are also mainly produced by East Asian newcomers, who are not very familiar with the older hosting platforms. Three Kingdoms initially enjoyed a huge commercial success, but then lost its momentum in an unprecedented scale, despite the overall positive reviews.
The situation has somewhat improved in January, thanks to the release of the Mandate of Heaven DLC, but given the limited scope of the pack, as well as the fact that it's riddled with bugs and glitches, I doubt that the reversal of the downward trend will be anything but temporary. In my opinion, the problem lies with the extreme lack of content, Three Kingdoms suffers from. Some of it is the inevitable result of the chosen scenario, but the business policies of Creative Assembly also bare some responsibility. Huge portions of the map are either empty or cut out (like Korea, Vietnam and probably Mongolia, as well), in order to be added lately as DLCs. Meanwhile, naval warfare was also removed, while visuals are quite repetitive (e.g. general portraits), despite the fact that the product was fully-priced. As a consequence of those shady practices, many customers, in spite of their initial excitement, grew bored of it and were not willing to try a second or third campaign, playing as an almost identical faction and fighting against largely similar enemies. All this probably explains why the fandom of Three Kingdoms has decreased so significantly, although the game was never a broken mess, like Empire and Rome II.
Therefore, if we take into consideration the appeal of Steam and Reddit, as well as the decline of modding overhauls and the fact that most newcomers that were attracted to the franchise by the lore of Warhammer and the appeal of Dynasty Warriors, are not accustomed to TWC, it's not difficult to imagine why the forum has become quieter. I would be very interested in sharing any statistics about the demographics of Total War, but, apart from the charts I cited above, I cannot find a more coherent and informative material.