I know for a fact that some 'long-standing members' who no longer post are...or rather, were...getting a bit bored with the forum and the same old/same old type of threads/posts etc.
Some subject matter just wears a bit thin after a while, I guess.
I'll expand that. I've been a member for 5+ years (and I believe I was a member before that, and rejoined) and my problem is finding something fresh to say. I know that sometimes I repeat things I've posted before. Yesterday I was thinking of asking a question on gear, but I used the Search function and found I had asked exactly the same question in the middle of a thread 18 months ago (it went unanswered, so I might 'bump' it!).
Added to that, there are times when I don't want to get involved in controversial posts or petty arguments, while quite often I draw back from posting because it seems I am answering just for the sake of it, so I keep quiet instead.
Sorry if this just adds to the negativity, but it's how I feel. In spite of this I can't keep away from the forum, I 'look in' a few times every single day.