Justifying ongoing U.S. military occupations by warning that China or Russia will stand to benefit from the ensuing power vacuum is illogical. There are a number of reasons to think that adversaries will struggle to gain meaningful influence following a U.S. withdrawal.
Jointness, meaning cross-service cooperation, is generally a good thing. But one can have too much of a good thing, and the Pentagon has too much jointness. Jointness in organizing military operations makes so much sense that the concept is overprescribed. Enhancing interservice competition for resources and relevance would encourage military innovation, civilian control, and economies in the vast Pentagon budget.
In Washington, military alliances have become an end in themselves rather than a means to security; an icon for worship, instead of a policy with costs and benefits worth weighing. Permanent defense guarantees inflate U.S. military costs, makes rich states into enfeebled dependents, and heightens the danger of getting pulled into needless wars. It should be obvious that U.S. alliances should serve U.S. security interests. But if alliances are permanent, U.S. security interests serve them.