I'd also like to point out that without even ATTEMPTING to identify "the problem" these statistics are pretty useless. And take that sentence itself with a grain of salt because as far as I can see, there is no problem, there is simply a cultural shift. Much like how MUDs are complete trash compared to modern day games, traditional forums have become somewhat cumbersome and unwieldy in comparison to social media.
God forbid we translate Bay12 into a twitter following lol, but there is something to be said for how sites like Reddit and Imgur organize posts, content, and interesting groupings in general. The appeal of DF is in part it's refusal to rely on visual clarity, apparent via endless upon endless tirades about menus, and graphics, and visualizers, but if you take a huge step back and really be objective about it, these forums look pretty early 2000s.
Though search query volume is something to be concerned about, it doesn't necessarily mean that interest in the game is fading, it could simply mean that interest is not growing as quick as it once was, and with famously long periods between releases that is bound to happen in an increasingly ADHD social media environment.
To go back to the MUD analogy, if you've ever played a MUD, you'll usually find a grizzled, ugly group of core players that has been playing for 30+ years, and likewise you'll find another group of spritely, young curiousos who are very much interested in the game, but can't really fit in with the now-ancient veterans, whose decades of in-jokes, house rules, and tastes fly over the heads of the youngsters. The same thing very much happens in any community, business, or other social grouping and certainly could be happening to this one. I think, in part, we have actually lost a surprisingly high number of core Bay12ers (RIP Girlinhat, the OG Bay12er that everyone knew) which, exacerbated by a high volume of more recent bans, has left the community feeling a little devoid. Some new members came in to fill the void, but were quickly rejected by Bay12 at large (See: Neonivek) -- and I'm not saying that it was wrong for that to happen, but we are now at a place where the most iconic members of an older generation are gone and the new generation has very few iconic members.
Heck, I remember when I was essentially the Neonivek of the community (I think) but lo and behold Bay12 has generally shifted much more towards my brand of humor/argument/general attitude. I've changed as well of course, probably becoming a little more aligned with how things were originally done around here.
The point is this: Bay12 is shifting culturally--as it always has, as we always are, and as most humans do in general--and though it has become more modern in its "site personality" it is still perhaps an artifact from a different time. It's really hard to predict what will happen to Bay12, The Forum, in the coming years. I think it IS safe to say Dwarf Fortress is no longer quite as famous as it once was, the articles have been written, the interviews given--what more is there to tell, until people feel it is time for such a gem to be rediscovered? If there is a major, major update then perhaps bay12 will be on the up and up again--after all, it seems most people discover the forums through the game--but perhaps it will simply slide into decay until someone wonders what ever happened to it all? And I think without reinventing ourselves, which carries it's own diverse range of positives and negatives--some quite extreme--we are mostly at the mercy of public curiosity.
There is no problem with this really, it's just life, and whatever happens most Bay12ers will always be able to say that the forums had a lasting impact on them, that this whole thing was a part of their life, and that the memories made here are for forever. Such is the way of things, and if you are a connoisseur of internet history, you may already know that such a thing has happened many times before to sits more famous than this one, and that it will probably still be happening after we are all long-dead.