It would certainly be true to say that a good proportion of Magnum's members were, have been or continue to be left/socialist both in politics and temperament. BUt it may be even truer to say that many were or are simply independent, even contrarian, by nature....
This question of PJ vs. fine art (or art-documentary fusion, as I have heard it called) is an interesting one. There can be little doubt that Magnum has shifted somewhat in that direction -- it has had to in order to try and widen (or maintain) its audience... But probably only in the same way that it has also shifted somewhat on the issue of corporate work (annual reports etc) and advertising/commercial photography. Had it not done this, I suspect it would be in even worse financial straits.
I have grown up with Magnum as a constant reference point -- not just in terms of what I value in photography and photographers, but also in how I think business can be organised and carried out.
I would be desperately saddened to see the agency renounce its social documentary roots in favour of the art market, but I would be even sadder to see it disappear entirely.
I suspect a bigger threat to Magnum than art photography is how it will continue to remain cutting edge and relevant for another ten, twenty, thirty years.
Even now I would be tempted to say that its best work may well be in the past. A good proportion of its members are reaching (or have reached) the end of their working careers and Magnum's wealth I imagine currently lies in its archive rather than in new work. BUt even having said that, I don't feel that Magnum will ever lose its place in my affections, nor those of many others who have grown up looking almost constantly at the work of this renegade co-op of photographers whose visual sense played a major part in making the twentieth century recognisable to us. Sadly, I don't think that is a feat they will be able to repeat for the twenty-first century.