Part of the reason why analytic philosophy generally is in such a healthy state is that, as Jerry Fodor observed in a recent book review, philosophers no longer tend to have philosophies. We no longer devote our lives to developing comprehensive philospohical or ethical systems. We are individually narrower and more specialized, which enables us to focus more carefully and minutely on the problems we study, and as a consequence to produce work that is more rigorous and detailed. The result is that philosophy has become more of a collective endeavour than it was in the past, in the sense that different people are focusing selectively on problems that are elements or aspects of larger problems. When the results of the individual efforts are combined, we may achieve a collective product that exceeds in depth, intracacy, and sophistication what any individual could have produced by working on the larger problem in isolation.
If nobody is devoting their lives to developing comprehensive philosophical or ethical systems, who is supposed to be doing the combining of individual efforts into a comprehensive collective product, or any sort of product at all? A collective product, like an individual one, has to be actually made somehow, and there is at present no mechanism in place for doing this if there aren't groups of people actually devoting themselves to it. Don't get me wrong; in intellectual matters I'm all for working together. Symphilosophie would be lovely. But somehow I don't think this particular strategy for it has been very well thought out.
My own view, by the way, not that anyone really cares, is that analytic philosophy is not in a healthy state, but is slowly collapsing, mostly due to serious inadequacies in the academic infrastructure that carries it, but partly also due to the fact that people are not really engaging in a cooperative venture -- people cannibalize and build on each other's work, but they do it mostly from scratch, again and again. There is simply not enough unity to have a collective anything; we are entering an era of Ten Thousand schools. And because of that, no one is really tracing out the ramifications of the detail-work people do (which in fact is only at the very top levels "more rigorous and detailed" than what people have been doing for decades now). This is precisely what would be avoided if philosophers took the trouble to have philosophies, and worked with those who had similar philosophies, but still specialized in particular areas; this is the only way I can think of in which the result McMahan is hoping for has ever actually come about: specialization inevitably collapses unless people have sufficient unity of opinion with each other to allow specializations to fit together and cross-fertilize. Moreover, I think a telltale sign is that what goes by the name of analytic philosophy has been steadily becoming more amorphous for decades now, with older terms -- like 'analysis' and 'analytic' themselves, to such a degree that it's difficult to say what 'analytic philosophy' is supposed to be -- being stretched farther and farther in a sort of concept inflation. This, too, has often been a bad sign in history, because it eventually reaches a point where all the major similarities and agreements are purely verbal and begin to be recognized as such. Of course, this is also often the very sign that people take to show that analytic philosophy is flourishing, using different words from 'amorphous' and 'concept inflation'; time will tell, I suppose. But at present it seems to me that such claims as the above have little more substance than a self-satisfied pat on one's own back.