So here's my take on this whole thing.... :)
Members come and go no matter if we're talking about forums, or Facebook, or car clubs, or churches, or pretty much anything made up of people. Why? Because people are inclined to change no matter how much they try to sell you on the idea that they are creatures of habit. It's just the way of life.
Folks say they drift away because the forum "changed" and I say that forums change because vocal, frequent contributors drift away and are replaced by new, different, vocal frequent contributors. When the guard changes by natural attrition, the forum changes. That's only a bad thing if the new guard is made up of complete a-holes.
I think we've managed to keep most of the a-holes at bay. Lately we cleaned house a bit and that seems to have made things better. ;)