My response is noted around contributing problems to maintaining empires:
The cost of defense was a major factor in the demise of many empires.
In a vacuum empires would take over what ever wealthy targets were available to them, Rome- the Mediterranean Bases, The Mongols – the lands bordering the Asian Steppes, The Dutch and Portuguese – the areas that there ships could safely sale. The last group you could call Trade Empires - although piracy was often as important as trade in establishing them.
But the vacuum that creates these empires does not last because of the “Red Queen's Race.” A phenomenon often discussed in biology, it also works with Empires. In Lewis Carol’s Through the Looking Glass the Red Queens Race had everyone running harder and harder just to stay in the same place.
When countries rely on some form of superior organization (Napoleonic Empire) or technology (Britain), at least some rivals will be able to copy the methods. And copying is easier than creating.
So to use Britain as an example, even though its fleet of 1914 was in an absolute sense, far more powerful than it’s 1904 fleet. The various lesser powers (U.S., Germany, Japan primarily) had caught up with it in the technical race. Britain ran hard, but still lost the race. In the end Britain was forced to pull its Pacific fleet to face off against the improved German forces. This ceded the Western Pacific to Britain's then ally, Japan. As we know, even close allies don't always stay friendly.
When countries lose the Red Queens race, they are usually (nuclear war possibly excepted) most vulnerable at the margins. The areas where there previously was a power vacuum can no longer be defended inexpensively, and there is a required pull back. Trade Empires seem to be particularly vulnerable to this extreme pull back.
So even if the United States manages to repair its current problems, and even prosper in some sense, that may not sufficient for it to maintain its empire.