Well, let's see if'n we can't get a thread started

When I got my first Day-Timer in 1987 or so, I'd have killed for productivity blogs and the like. As it was, there were a few books (Lakein, Bliss, Smith, Hobbs) that your local library may not have had, but they were pretty generic or didn't address topics in the detail I wanted. So I felt really left on my own trying to figure out how to use this stuff. (And it may be the case that my life was simply not busy enough to deserve such tools.)
I found DA's web site when his "forum" was a single unbroken thread of posts on all sorts of topics. His book broke at a time when the networks on the web were really starting to feel their oats, I think, and people had all these tools, lots of work, but no idea how to use one to help them with the other.
I think lots of us were intrigued by this shiny new toy called GTD, his introduction of various concepts, and most importantly, a full-blown, pretty well thought-out, and mature system and philosophy for handling the flood of non-physical tasks our flesh is heir to. He had The Answer. And so, many were the disciples (self-appointed) who started blogs, wrote essays, started a movement and then, once they started hammering on the system, saw that it didn't really look as good at home as it did in the store window.
(This is not to be cynical or anything, because I sure did the same thing whenever I found any book in the 80s and 90s that promised a way to manage my life's chores and tasks. I found a book one time called FILE...DON'T PILE and totally changed my filing methods accordingly, very enthusiastically, and then discovered what a pain in the ass it was to maintain and dropped it.)
For myself, I took up many of his suggestions, but could never get the altitude metaphor, it just never clicked with me. And when I had my lists on my Sony Clie, I also obsessed over the lists. I looked to the lists to have The Answer to what I needed to be doing, and would start fretting if they were empty.
I think we're all intrigued by the new, and let's face it, DA's work provided a wonderful focus for people's creativity, to think about these areas of their lives. I think one reason the web sites and blogs have died out is, yes, there are only so many things that you can say about this material and you do get in the echo chamber.
But also, isn't it the case that when you're learning something new (or falling in love), you throw yourself into it shamelessly, wallow in it as much as you can, learn as much as you can, examine every crevice and pore to find the truth wherever it may be? I've thrown myself into fiction writing, theater, design, etc. at various times of my life. Many things. Some of them didn't turn out to be long-term engagements. But certainly I learned something from all of them that I carried with me to my next obsession/love affair/interest/hobby/etc.
I'd say the chatter has died down because people have learned many wonderful new things about managing tasks and themselves, but the absorbedness they had in the subject has died down and and they're ready to move on to the next obsession/love affair/interest/etc. It's been a fun ride and we all had a great time. And, surely, everyone who got on board learned something that they're carrying on to their next gig, yes? So we can't knock DA too much; he did tremendous work in getting us started, now we have to take ourselves the rest of the way.
For myself, I moved on to Mark Forster's work in 2006 and have found his DIT system more in line with my way of thinking and doing, though even there, I find DA's Projects list and Waiting On list pretty vital. My home desk's inbox is overflowing with stuff from the time the fall semester started, but all urgent and important stuff has been taken care of, so all that's lying in there is stuff that can wait.
I also can't help but think that another reason the GTD channels have quieted is that we're asking ourselves, "Now what? Yes, we have the tools, we have the knowledge, and now we're productive -- but for what purpose? To what end?" Hence, Merlin's shift in emphasis, which is coming right on time, I'd say.
DA, after all, doesn't make a living being productive. He makes a living selling stuff, teaching stuff, and writing stuff. All those fancy black-belt GTD moves can do for him is to help stay on top of the selling, teaching, traveling, writing, etc. activities.
Our trick is to move from apprentices to journeymen and find our own activities to apply the GTD moves to.