Having read through this thread I think the perception of reduced activity is probably the result of everything mentioned thus far and more. Undoubtably the growth of other digital platforms, IG, etc., will play into some collectors shifting interest. The way threads are presented in the CGC's 2017 redesigned platform ...without readily displaying multiple responses which made the earlier platform more conversational and fun to use... might be a factor as well. In general, do other forums and platforms compete for folk's attention (time spent here)?
It also occurs to me that there are two competing third party grading services, both with active message boards. One of the two boards allows for the posting of slabbed comic images from both, the other has a policy against it. Whether this is a factor in diminished board activity is anybody's guess. One solution might be for the CGC to review their current policy disallowing collectors from posting images of graded books with a competitor's third party label. Logically, CGC shouldn't be concerned about anyone else's label since they consider their holders & service to be the superior product. Just my 2 cents; other's mileage may vary.