I might also mention the mighty Sony, the PS2 which roundly trounced both XBox and Nintendo the first round has a pitiful replacement in the PS3 which XBox 360 triumphed over, also the Wii is now leading the pack handily--a come from third place win.
So no, monopolies are not unassailable. It's just MS knew how and apparently no one else has twigged to the trick yet--except maybe Nintendo.
Certainly not the penguin, that's for certain.
