Thank you for sharing your thoughts. I have experienced some of this as a mod here. One comm is okay for now, because the small userbase is very very committed to keeping it and posts are irregular but they are there. The other one just fell asleep at some point and there never was a lot of activity to begin with.
But I had never seen this comparison to LJ (as I was never registered there, only lurking) but I am sort of relieved to hear that that‘s not a Dreamwidth only problem. I always attributed it to the increasing fractionalisation of fandom....

ETA: I should add that I really enjoy fictional_fans, especially because of its panfandom nature.