We've lost so many posters over the past decade. Sure, some have passed away, others have deliberately moved on, but
I suspect many have simply "faded away" - they are still officially members here, and could log-in and post(if they could either remember their P/W or figure out how to recover it - something not exactly obvious on this site).
Alex, surely you have, somewhere, the email addresses of The Disappeared? Why not send out a one-time email to all the missing members and remind them they are still members. Perhaps spice it up a bit with (something like) "There have been 17,692 new posts since your last visit" and helpfully provide a link to where Silco members can recover their passwords? I'll bet some of them will come back. Nothing to lose, and everything to gain. Doesn't cost anything to try.
If it's a time-constraint/labor-intensive issue for you, I would volunteer my time to help sort/do the emailings. I'm sure Woodsy would chip-in, as well. What say you?