Yep. A traditional forum ages and grows old. And as they get older and older, it becomes harder to draw new members because of the clique of the core membership. I've seen a few traditional forums die that death over the years.
And some forums, and I belong to several, the members are literally dying from old age. We are all mostly old and retired. And we lose members every year due to death. Several times a year there is an obituary post for some long time member.