what if Microsoft decided to close down the Operating System side of the house. In other words they would shut down “Windows” OS, but maintain the rest of the software and hardware divisions already in place. Now to ensure that the two present systems (Vista and XP) continue on they would agree to keep open a small portion of the Windows teams to ensure that all problems for the next three years would be covered, however they would no longer work on a new OS.
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Under your premise MS would still maintain/support Vista and XP, just not create any new OS. There really is no need for any of the other questions. Vista and XP would still be the dominate OS's out there on more computers then all the other alternatives combined. Even after three years, if it ain't broke why does it need to be fixed?
Now if your premise had MS going away completly, different story. Well maybe not, if MS goes completly away chances are they would sell the code, maybe even the rights to the name. Think about that, even if Apple brought the code, it would still be Windows.