Only they haven't.
The reason the US and Russia haven't run rampant the last 30 years is because the US and Russia can blow each other away. So Russia satisfies itself with Georgia and now Ukraine. And the US satisfies itself with those nations in the MidEast region. Remove Russia, and the US would have been all over the yard. Remove the US, and Russia would have been all over the yard.
And now with the rise of China, mankind is even safer. Not only that, but more of mankind have begun to enjoy the benefits of safety. The US and Russia were perfectly willing to fight a proxy war in Korea 70 years ago. They wouldn't dare do that today. Why? The Chinese presence in any MAD calculations.
Today Africa is much safer, it is developing much faster. A large part of the reason for that is the presence of China. Now instead of proxy wars, the great powers compete to see who can deliver Africa the most undersea bandwidth for instance. This is a much better situation for Africans than the situation that existed when only the US and Russia were superpowers. And this situation exists because of MAD. It proves that an armed society is a polite society.
The US, China and Russia are all fulfilling their responsibility to keep the peace by maintaining lethal arsenals of weapons of mass destruction so that they are each confident in their own ability to destroy the others. And that is good for all of us "little people" in the world. Whether we are American, Chinese or Russian. (And even good for us if we are Korean, African, or from South America.)
