A summary position is that -
1) The US created the post-WW2 world order - the global free trade system - as a bulwark against communism. It was left the only superpower standing and did the smart self-interested thing of preventing communist takeover of a war broken world.
But then Eastern Bloc cracked and crumbled with surprising suddenness in 1991. The US had won the Cold War but then failed to figure out how to cope with the peace. It was again the hegemon by default. Much more so even than after WW2 where it still had to dismantle Britain and other still colonial empires.
A succession of weak presidents meant the US was a world leader without any particular world vision. The situation became divided into a "Davos elite" hoping to continue onwards with the "globalisation project" towards some kind of planetary governance based on the kind of techocracy that is at the heart of all actually successful modern democracies, and then the US blundering on with an increasingly domestic focus on its interests.
It didn't actually want to lead the free world. It just wanted to be free to do its own thing. Burn oil, eat junk food, gamble on markets. Party it up.
(Of course, the technocratic part of US society wanted the opposite. But their moment had passed with the Cold War challenge too).
2) The world has moved on towards some kind of next step. But China can be discounted as a major player. It is a bubble enterprise tied to the free market world order that the US created and continued to underwrite even after it had lost its main security purpose. China matters as part of the much more important story of a technocratic/democratic Asia. South East Asia’s 2.5b people beats China for population and its GDP should match China by the end of the decade.
The logic of the situation is that the US is going to turn inwards on itself finally. It has so many geopolitical advantages, it simply doesn't need the hassle of trying to run the world.
The US has the world’s best chunk of geography. It has the best chunk of food growing land and an ideal range of growing climates. It has an isolated position that means it never has to fear rowdy neighbours or physical invasion.
It has demographic power too in a population of 330 million that isn’t greying dramatically like all its rivals. It has energy abundance with its shale oil and gas, plus the easiest transition to a practical renewables infrastructure.
It has - as
@ssu underlines - the dollar embedded as the world reserve currency. That is an incredible economic advantage that will be tough to unwind. It also has now tied in Canada and Mexico as its North American alliance - Mexico as the replacement China manufacturing hub, and Canada as yet more resources and growing land.
So nothing stops the US curling up within the comfort of its own North American empire and saying the world can go f*** itself. The inbuilt advantages are so many that even really bad political leaders can't actually sink the ship.
In this scenario, the US is no longer the world leader - except in the various ways it might still want to get involved in running other people's affairs.
The desirable outcome is a world that continues to globalise - but only via a more intense phase of regionalisation (the view
being pushed by Parag Khanna for instance).
So Khanna talks of an age where we move on from hegemonic states - single nations running their respective empires - to regional power networks. You already have Europe as a reasonably integrated system - organised in its own "typically European" way.
Likewise Asia will emerge as a geographically organised community of interest. Belt and Road could be an important part of that integration, but China will not then "own" the region as a result. It becomes a large component of a more general workable identity - depending on which way the CCP go.
Then the US as North America is another regional bloc with its own political flavour.
Out of this rationalisation of world geopolitics might come a regionalisation that makes a better foundation for globalisation. Instead of the rather Western model that the US sought to impose on the world - for security reasons - there would be the opportunity for something more inclusive of the way the world actually is.
Of course, the problems of the world may fast overtake the political opportunity to grow that world-level of governance. But there you go.