while all the scenarios you describe above are plausible and somewhat likely , there is one scenario that you have not investigated . people , companies , nations - everyone runs out of steam at some point . the (western) roman empire did fall . GM and IBM did falter . sooner all later , the dominant force yields , either to a new rome or a dozen barbarian tribes .
there is an argument to be made here that microsoft has already peaked and is at the beginning of what is likely a long and more or less graceful decline in power and influence . perhaps the fate of IE is an indicator , as are the interminable longhorn delays and functionality amputations . what is clear at the moment is that google and apple are -right now- at the top of their game , and MS is not .
things can happen . hit the couch and grab some popcorn .
The Gathering Storm: Waiting For the Microsoft Beat Down