I remember in the 'old days', when IBM came out with a personal computer, many businesses just reflexively bought them. Those lucky (unlucky?) enough to have IBM boat anchors in their buildings got some free, and even got huge percentages off of them. Many students also reflexively bought their PC's too because that is what they used in school.
And there are still people that will ALWAYS buy a 'pc'. They will always use Windows. And they will usually, always hate Apple and the Macs.
Microsoft will likely always be around. They might shift to a Linux core, but they will likely always be around because there are people that will always buy their stuff, and love it. Just because that's what they have always done.
I think just as many people have been prophesying the end of Apple as have been prophesying the end of Microsoft.