Very interesting discussion. I'm almost a 100% lurker, but I'm going to throw one thing into the mix that's completely personal preference:
I've seen this in a number of forums I used to frequent....
The look of the forum is actually visually exhausting. Many internet forums use to have clean and simple page layouts. A forum thread actually looked like a neat and clean conversation.
Today, the look is visually cluttered and exhausting: colored gradient gray bars on the top and bottom of each post, quotes are white text in dark blue boxes, the posters name and avatar are in a black thick box, there are dark colored logos and signatures all over the place, etc.
What this actually fosters is the online version of a conversation overrun by noise and distractions. Instead of the actual conversation and photos taking front and center stage, my brain is pulled in all directions looking at colored gradient bars, dark blue boxes, dark black boxes, logos, huge swaths of color, etc. Yikes!
I know this sounds silly, but I know of a number of forums that switched to a visually busier layout over the years, and the number of quality posts went down together with the new "visual update". Yes, that's anectdotal evidence, but I feel it's true, and it feels true.
When the forum layouts return to a simple place to have a conversation with a CLEAN uncluttered look (which also loads faster), I'd wager that the conversation would pick up again. ....but not from the old timers. In my experience, once someone stops posting, they rarely go back to becoming a frequent poster. I have theories on that, but that's a whole 'nother subject. But new comers might be encouraged to carry on coversations.
...Just a thought.