I agree that open source is not about to roll Microsoft back on the desk in the near future. But it is sufficient to keep Microsoft from nailing down several more profitable monopolies, and reduce some of its profits from the current one.

And in the longer future, well the history of disruptive innovation suggests that companies which are about to get wiped out have their brightest time in the sun just before their markets start vanishing from under their feet. Whether that will happen because of open source applications eroding Microsoft's market share, or because of something else, I don't know. But Microsoft is going to encounter something. It always happens. (And I don't see any reason to believe that open source is not the kind of disruptive innovation that Microsoft should most deeply fear.)

Cheers,
Ben