I can’t see why English would lose its leadership. USA is still the most powerful country in the world; many people speak English as their mother tongue; many people speak English as a second language; English is the language of science, technology and business.
I’d love Spanish to become the leading language in the world, as it’s the language I speak best, but I think that one of the reasons for its growing importance is that Spanish speaking countries are in America: Spanish is the most widely spoken language in the continent, and so, people from the USA are willing to learn it. USA spreads it’s customs all around the world, for example Coca Cola, Hip Hop and the trend of learning Spanish
As for Chinese, it would be a terrible waste of money if we had to change all the scientific and technological things that are written in English, and in latin alphabet.
But I’ve studied History, and they say that historians are completely unable to foresee the future. And I agree with it; anything could happen: Polish will be the next leading language in the world!