Having created, by accident, the empire, and having done so for the purpose of exporting our market system, and its trade routes, we are stuck with the very real consequences of creating power vacuums if we withdraw our military power, and create opportunity for the greater cost of NOT acting as we are acting.
We have, after all, made a nice profit out of bringing the Hindu and Sinic cultures into the modern era. We have, and continue, to make a profit bringing the Islamic cultures into the modern era – by exporting debt (that we may questionably have to pay for) rather than by collecting tariffs or taxes for having done so. These efforts have been made under the rubric of political democracy for the purpose of popular opinion, but are actually for the institutional purpose of creating an economically incentivized and politically enfranchised middle class that is invested in perpetuating the world market system.
I do not think that there is disagreement among political economists that we would be better off without having to support the empire. But when faced with the very real, and very negative impact that a withdrawal would have on the average (pampered) american, and on the average (schumpeterian) public intellectual, practical heads prevail.
As I understand it, the general thinking among the strategic thinkers (those who study military, political, and economic relationships, rather than just political, financial and social relationships) is that if we bear the burden long enough, the world will evolve into a sufficiently middle class economy (a synonym for democratic) that the purpose of the empire will decline at a rate equal to the relative importance of the american economy, allowing us to withdraw without creating shocks to the international system.
A failure to understand military history is what separates ideological political economy from practicable political economy. Property rights are indeed the basis for prosperity. However, property rights are an institution that is created by the application of organized institutional violence.
This fact is usually lost of ideological libertarians.