We're going to be the world's leading superpower for a lot longer than anyone thinks. I don't think China any longer has a chance of catching up to the United States as a world superpower, since it appears to be turning into a giant version of 1990s-era Japan. Once you slip into that trap, it is awfully difficult to get out of it, especially given adverse demographics compounded by China's self-inflicted one-child policy.
But unlike Japan, China has a substantially larger population and substantially smaller GDP per capita than the US. China's so far behind the US in GDP per capita, that even in a pessimistic scenario, it still grows faster than the US, doesn't it? So it keeps closing the gap on the US, it's just that it'll take a lot longer to reach parity than previously thought possible.
I don't see where China will get the hydrocarbons to continue growing arithmetically for 84 more years. At some point, the world will hit hard limits to growth. To answer the question in the OP, I expect no one political entity will be able to exert global hegemony in 2100; the most powerful states will be regional hegemons.
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