Just as there are theories about how the Earth will eventually meet its end, so there are theories about what will happen to Microsoft someday. Not that the death of a software company—even the software company—is very significant compared with the loss of ones planet, but whatever happens to Microsoft could actually happen in our lifetimes. Almost by definition, the end of the world will not.
I mention this upfront to demonstrate that there are worse things than a world without Microsoft. Some would say there are few better things than a Microsoft-free environment. I wont get into that discussion today, instead concentrating on the challenges Microsoft faces and how they play out over the long term.
Challenges from below—I used to think the big threat to Microsofts dominance would come from an easier-to-use PC that would change everything. There was a time when it looked as though a game machine might eventually grow into a general-purpose home computer and, eventually, into a general-use desktop.
There is still a possibility, even a probability, this will happen, but its not going to happen very soon. But I remain unconvinced Windows will ever be the operating system that makes personal computers both ubiquitous and loved by their users, and that means there must be something else that accomplishes the goal. Maybe it will come from Microsoft, but maybe not.
The differences between hardware platforms (game boxes vs. PCs vs. media centers vs. whatever) have become increasingly slight and will disappear over time. Home media servers will appear, and real broadband-to-the-home (100M bps and up) will change how both information and applications are delivered. These things also threaten Microsofts dominance, but this is a 10-year horizon, and much can happen.
It should also be remembered that Microsoft is in some ways a stronger consumer products company than it is an enterprise player. That could create two Microsofts at some point in the future, one for each market.