Did you read Asimov's "Foundation Trilogy"? The Galactic Empire continued, indeed thrived, even while the periphery crumbled and broke away. The megalith called Microsoft will continue, even while Apple and Linux and others eat away at its empire.
Microsoft is not dying, not by a long shot. Windows, or whatever Windows becomes in the next few years, is so much a part of how companies do business that it would take the collapse of American business for it to get loose.
I have doubts that anything can take Microsoft's place in the enterprise anytime in the next two decades, maybe more. And what does take its place will likely be an outgrowth of Microsoft, not something else. Certainly not Apple or Linux, and nothing else is on the landscape.
Now if you were to ask me whether I like that or not, well that's a different question. I'm just commenting on the degree to which Microsoft rules American business, not whether I personally like that fact.
In light of some things Microsoft has been doing since Balmer it may get to be liked a lot more.