I think that the fall of Microsoft is eventual anyway, no company has historically maintained a strong dominance or monopoly in their industry over a long period of time. As things continue to change in the computing market, Microsofts sheer size works more against it than for it. We've all probably heard the story about how Microsoft almost missed the Internet in '95 or so. That one was easier to recover from than sinking billions of R&D dollars into a desktop app development when things suddenly go to apps in the cloud, or some similar thing.
At this point, I don't know that Microsoft can really benefit from any significant merger. They may however be able to fend off the inevitable for another few years with some strategic acquisitions though.