J-O, if you are satisfied that dwindling numbers is a true trend and not a temporary blip, why not consider incorporating the pages into the Forum? That way they would be live and easily available to both those searching only for one specific topic and casual browsers. The latter, in particular, quite likely visit the forum looking for one thing, then stay because they find so much more of interest, but don't become aware of the main site contents. It's like it used to be in the days of the encyclopedia (book form). I would go to look up one thing, then become so beguiled because as I flipped the pages other fascinating things would catch my eye. In the end, realising the time, I'd shut the book and go no further.
One point - it would mean sacrificing those topics that have no or little content, the ones for which you have appealed for help in the past. When you once mentioned deleting them, I remember suggesting that as they do no harm where they are it would be best to leave them in hopes that one day someone will come along to write or complete them. But if all the good stuff there is on the main site were brought over here, it would enrich the forum - not that it needs it.
"To thine own self be true.......
Thou canst not then be false to any man."