You make a faulty comparison. Britain eventually lost its status as a great power as a direct result of the First and Second world wars, having been bled dry of both men and treasure as it battled Germany. Who knows how long the British Empire would have lasted if the Archduke Ferdinand was not assassinated, and neither war were fought?
That's not the only reason. There was also increased desire for independence in many of Britain's colonies (India, Africa, etc), and other empires have fallen for other reasons. Spain's possessions in South America disappeared partly because of its overreliance on gold and silver from Mexico and Peru (think what will happen to Saudi Arabia once oil runs out), more than native revolts.
Also, who knows whether there will be a World War Three? It's totally correct to say that China is starting to march its influence into Africa and Latin America and binding together with Russia while strengthening control over parts of Southeast Asia (Burma, Cambodia).
Not now, of course, but by the middle of this century China will have become a moderately developed country having surpassed the US as the world's largest economy (even this is a weak assumption; will China fall into a sluggish recession like Japan in 1990? When democracy comes to China will there be mass social unrest like in Indonesia?)
I think it's safe to say that the US will no longer have the title as the world's undisputed superpower.
As for other parts:
Europe has half a chance of becoming a superpower IF it can get itself together (which is not likely now), Russia might be a medium-sized nation with considerable influence like Germany, and who knows? Maybe other regions will fuse together before then and prosper.
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