if some people migrate to mac or linux, so what? they will be back sooner or later because of some compatibility issue or other.
Its not some people, its millions of people that are switching. And since leopard came out, its gone sky high. And a good amount of it can be tribute to Microsoft's failure to innovate. I can't think of anything that they have innovated on since windows 9.x
There code base is way too big, and total crap, and they can't innovate on that. Look at there mobile platform, its total garbage, hasn't really been changed since v4, apple comes in and spanks them, now google is going to come in and spank them some more.
The xbox is about the only thing they have done right in the past 5 years, and look how messed up that has been.
Not being able to innovate will be the death of Microsoft and its already happening. They are stuck in this 1980's mind set, and steve ballmer is going to be the worst thing to ever happen to them. He will be the one to kill Microsoft because he has no clue how to make his developers innovate.
EDIT: The biggest reason Microsoft falls to innovate now is because they are too big. All the teams they make for project's are too big and they can't seem to innovate when they do that. The first xbox team was very small and they did innovate with that.