It's interesting that, if we think little, practically everything is a monopoly. Every hotel or corner grocery has a monopoly in virtue of its specific location. The author of a book of poetry has a monopoly on its publication.
As for America losing its hegemonic power in the world of politics, it could be argued that nothing would be more beneficial for the future of the world. Some libertarians, especially conservatively oriented ones, think of libertarianism in terms of one vastly superior power (national or international) that can impose its will on everyone else; then the point is just to make sure, somehow, that that power is always held by the good guys. Right. I agree, on the other hand, with Leopold Kohr (The Breakdown of Nations) that our security and freedom lie in a homogeneous distribution of political power, nationally and internationally (if such concepts are retained). That seems to be the precondition for anarchy, as in medieval Iceland or contemporary Somalia, with no one entity being able to dominate the others. I'm much less concerned about Bill Gates' de facto monopoly in software than about the government's legal monopoly in police protection.