Activity is, measurably and objectively, not decreasing. Pretty much all activity - new topics, new members joining, overall replies, etc. - is steadily on the rise.
I understand that it might feel or seem like things are slow in some forums more than others but, speaking as someone who can see the actual data behind the scenes, the forum is on track in the big picture.
If the mods did start creating threads to spur activity, you really don't think people would start complaining, "See, this place is so dead they have to have the mods post just to get a discussion going". C'mon. I get what you're saying, but I believe that would be a quick walk into a no-win situation.