I think it's a combination of factors. The Discord thing I definitely think is one. Certain types of communities seem to be more inclined to shift to that as opposed to others, and this one comes across as the type that may be more likely to shift in that direction. As far as general activity, I've always been too much of a background lurker to be able to comment as to whether it's gotten more dead or not, but it sort of "feels" like its died down lately to me.
I think the video pace may also be a contributing factor. Each release is potential for new blood, but with how infrequent things may come out, that works against things in that manner (though the pace comment isn't a complaint; I'm definitely a proponent of "when it's done"). Maybe if in a perfect world the popular AF series like Freeman's Mind and CP were being banged out on the reg, that would increase chances of catching new people with each release, but again, forums seems to be very slowly becoming a more "antiquated" format for communities like these, and aren't as attractive to some people.
I imagine the forums here probably wouldn't go away (I was part of one that eventually shut down since there was like 5 active people left), but lack of activity definitely sucks for any forum. Barring new visitors, really the only thing that ever keeps stuff like this going is the frequent participation of the mainstays, though that eventually just turns the place into a giant inside joke that can tend to alienate new people.
Who knows. I think the best solution is going to be taking the enlightenment van out to town and bringing some fresh acolytes back to the barn.
Say no more, already sold.