I'll put my 2 cents in.
Firstly Jean reminded me of Sheldon from Big Bang Theory - no need to elaborate there I think. It is a loss, but I heard a story once in an IT firm that relates to this and offers some hope.
There was a team of 5 developers, one of which was clearly more tallented than the others. The projects essentially rode on his back. He knew that he could code rings around all the other developers put together and became very difficult to deal with. He was frustrated that anything complicated came to him so his workload was through the roof and the other developers were frustrated that he was such a difficult person to work with. He kept saying that they needed an entire team of "people who could program" (i.e. at his skill level), and they kept saying they needed a better way of working as a team.
To everyone's surprise, the company let this "superstar" developer go. That one key developer was snapped up by another firm and the team initially freaked out. They hired another person to fill the space (although the other developers thought they should hire another 2 or 3 developers). What happened was actually quite amazing. Productivity increased!
It turns out that those tasks that nobody else could do but the one guy had to be discussed as a group in order to resolve it, meaning that all the group was then capable of those tasks. What's more, as the team worked out how to do all of the different tasks they became better problem solvers and better programmers in the end creating the team of "people who could program". With a more pleasant team environment everyone started working with eachother much more productively and actualy enjoyed their job.
I think the same thing could happen here.
For instance, many times I saw an interesting question fire through only to see someone had jumped in after a couple of minutes with the answer. The dominance of a few who had the right answer immediately removed any chance for the forum to discuss the issue, formulate ideas and develop a consensus. Nobody learnt anything - they just got answers. What's more nobody got a chance to contribute an answer unless they happened to be logged in at a time that those key people weren't on.
So what's going to happen now that JMG, Tom, Valery, Stuart post less or not at all? Chances are that they will be very soarly missed and in the short term the community will be that much poorer. But the community will keep going and in the long term may actually strengthen.
As for Jean's frequently abrasive language - what about freedom of speach? I love this quote from "The American President"
"You want free speech? Let's see you acknowledge a man whose words make your blood boil, who's standing center stage and advocating at the top of his lungs that which you would spend a lifetime opposing at the top of yours. You want to claim this land as the land of the free? Then the symbol of your country can't just be a flag; the symbol also has to be one of its citizens exercising his right to burn that flag in protest. Show me that, defend that, celebrate that in your classrooms. Then, you can stand up and sing about the "land of the free"."
