USA's place on the top of the economic pyramid isn't a given. Nor should it be. There are other much more populous countries, who should grow, to be able to have very high standards of living. If the USA keeps on top for the next 50 years, our standard of living will far outpace most other places in Asia / Africa / Central South America. I'm not sure that's a good thing. Even if China's economy surpasses that of the USA, that economic strength is watered down over a much larger population. 1.35 Billion vs. 340 million. China's economy would need to be almost 4x greater than the states, to have economic parity, according to population.

Will China be on top though? That's hard to say. China is great at copying technology but has been challenged with creating high quality patents - innovation is still a struggle for them. They have a ton of huge issues from environmental challenges, to a rapidly declining population, to an emerging generation that is not as driven as the previous generation. If they can figure out how to innovate, on the scale that the USA innovates, they'll do well.

The USA has a ton of issues as well, including emerging class conflicts and extreme class income disparity. America is politically and ideologically polarized, and drugs and crime are spiraling out of control. Unless the USA figures out how to address some of these issues, the USA is in for a world of hurt. Our most progressive cities are a mess - San Francisco, etc. - and those are the policies toward where the rest of our cities are moving. America still has much more going for it, but in 50 years, it'll be interesting to see how all this shakes out. There were huge changes in the 50 years from 1920-1970, then tremendous changes in the 50 years from 1970-2020, who knows what'll happen from 2020-2070.