Interesting question. I've actually been reading this thread for a while, but haven't posted yet since I have not visited the discussion forums in months (I post in Esty's RPG when I have ideas, check out what's going on here so I'm not totally clueless, and that's about it). So I've suspected that most people will just jump up to give me a lecture on never saying that there's nothing left to wonder. Personally, my only reply to that is that while life and the world contains infinite things to discover and discuss -- Tolkien doesn't, and to imply that his works are truly limitless is to think he somehow achieved full God-like creator powers.
He is a man who wrote an amazing legendarium, which will still never compare to the wealth of ancient mythos from around the world and all the different fictional works of the modern era when taken as a whole, which in turn will never compare to the ever changing mysteries of non-fiction a.k.a. real life. There is a good reason there's a cliché that says "variety is the spice of life", which is why I'm very reluctant to agree with those who say you can never devote too much thought to the Professor. As a collective site, maybe not, but for each individual member, I think there is a limit. Some have a much more voracious appetite than others, but still.
I'm not as interested in delving into it as I once was. But that is purely a personal change of inclination. The Barrow Downs will keep getting new members who are fresh and excited about Tolkien (as has been said by others in this thread) and so as long as they're discovering new things about his works there will be things to discuss here. Maybe somewhere at some time they have been discussed already, but not by the same minds, and that is what should matter. So I see the whole issue of there "not being much left", as applying more accurately to personal feeling -- for some people (and I happen to be one) there is a point you reach where you realize that you don't
want to know anything more about Tolkien, not because you think you know everything (I haven't even read HoME) but because you want instead to learn about something else entirely. I only have so much time in the day and so much head space to use.
I mainly devote my computer time to my original writing and writers' group, and my writing is far more inspired by other authors whose works are "off topic" here, so hence the lessening interest in dissecting Tolkien as time goes on. All strictly personal views and reasons.
So I see what Imladris means about expanding. But I don't really see that happening to this site: it is a Tolkien site, and as such is inherently limited to a very narrow list of subjects. It would have to change its very nature to "expand" into something that no one will ever tire of or feel has been personally exhausted. And why should it change? The fact that some members drift away is the nature of the beast, and there is no shame is lessening interest. That will happen whenever you narrow something down to a certain subject, even if it's a broad or deep one. There will always be a point where you get "off topic", and that can feel stifling when you start to discover different things you're excited about. So you would have to remain
really into Tolkien to be able to keep finding original things to discuss about him, day after day, week after week, month after month, year after year. Some members will return after a time, some won't, but the BD doesn't need to change to compensate since it gets reborn with every new batch of eager minds. Things would be much different if it was a small site with a mostly unvarying base of members. Then I think it would have to change with those people to stay alive. As it is, there are other sites and other forums for discussing other things, much as it may be painful to be a Newbie somewhere all over again. But such is life.
I'm not sure why I wrote that dissertation on a subject that's been well discussed (ha, déjà vu!) except that I've always been fascinated by the nature of growth and change, in people, places, etc. When is it good and healthy to move on or alter your purpose, and when should you stick to what you've been doing or believing? Perhaps that's too broad a question though, as it's practically the same thing as the debate between fundamentalism and liberalism. So to put it a different way, what is the difference between a fad you shake your head at later, and something that, while you may not always be as deeply engrossed in, you will still count as time well spent and a valuable learning experience? I know I stated one difference in the phrases "shake head at later" and "valuable learning experience", but both of those still have lasting impact on your memory, so there's probably less difference than you'd think.