Successful Forums: How/Why can they Fail?

Let's say that you own and/or manage an extremely successful, consistently active discussion forum that is renowned for its excellent discussion and friendly community. As the forum grows older, you find that the activity levels drop dramatically, members who were once active stop visiting, and new member registrations have come to a halt.

In short, what are your thoughts? Why do you believe that "once successful" discussion forums can fail miserably, even after lengthy periods of stunning activity?

 

As a member of various forums, sometimes I find I'm spending too much time on them and decide to cut down on my usage. Maybe an established, busy forum will have a lot of users who make the same realization. That's my idea anyway.

 

You make a valid point, however...

Let's say that one of these forums decides that it's time to "spruce up" their visual appearance. If they were to go about doing so, would remaining active still be viable to you?

 

News of a new template might be enough to bring me back to take a look in such a situation, but if I were still concerned about my time usage I would only be tempted to fall back into my old habits if I saw some interesting threads.