Russia, China or the Caliphate will step in and lead the world.
The only reason the US has slipped is that our enemies, mostly domestic, have steamrolled over the Constitution that was engineered to keep us above and beyond the rule of man.
China and Russia aren’t as strong as people think; their respective economies have the illusion of strength, but if you look beneath the surface, you will see in the case of China a massive credit bubble (and credit bubbles don’t end well) and in Russia’s case a kleptocratic economy that is far too dependent on resources exports; Russia’s economy lacks diversity.
In the case of a caliphate, there simply isn’t one. The last caliphate sputtered to an end when the Ottoman Empire fell in 1923. Turkey, in theory, could be a major leader in the Islamic world, but with its fractious nature - not to mention an ongoing transnational Sunni/Shia religious war, akin to Europe’s Thirty Years’ War - the Turks would have far greater success in herding cats.
They have the potential to be a leading Islamic power, but it won’t be with Erdogan; he’s far too much his own worst enemy. And it won’t be Fetullah Gülen either. I don’t see that happening for another decade at least.
The Caliphate is on the move and now has atomic weapons. That behooves those of us with MIRVed thermonuclear weapons and lasers to keep a sharp eye out.