I see what you are getting at but I have to disagree with alot of what you are saying.
> the US will find itself in the same position the UK is in now
The US and UK are totally different animals at the height of their "empire". The UK currently has little land and alot of it is devoid of minerals(sans coal). They achieved most of their might subjecting other countries and extracting resources from their colonies. Once the colonies and the rest of the world objected the British empire began to crumble as the colonies broke off from the UK.
Contrast that with the US which if it were to jettison its "colonies" (Puerto Rico, Samoa etc) you would see very little drop in GDP. The US has vast swaths of land which is excellent for farming, energy extraction and contains valuable minerals. It has widespread and diverse populations with wildly different ecologies and climates. In addition to a huge amount of resources it has a kings ransom of some of the best universities in the world(stanford, harvard, MIT, etc) and a highly educated society with living standards just below some small asian and european countries. The US also has a peerless military (we need hypersonic weapons however) and many aircraft carriers and nukes as well as world renown special forces groups. And lastly of the top largest 100 companies in the world, 65% are US based and this despite the US only having 4.2% of the worlds population, looking at GDP per country is almost comical as the US has 30T compared to the next largest economy China(19.5T), 3rd place is Germany at 4.9T.