To me, it is not a question of whether it will be replaced but rather when. Another language will take its place eventually, but the English might evolve so much that we won't recognize it as English when that happens. Somewhat similar to what happened to Latin. Of course, the written mode being now everywhere slows that natural process a little bit but it won't totally stop it.
Many people think Chinese is the most likely to replace English. I don't think it will. Although, it is the most likely to replace English of all the languages in this current world-state and context, I think there are factors that will prevent that from happening. Chinese is not as accessible as English because of the script. Of course it is possible to learn, but for a language to develop into a world language, it usually has to become a lingua franca - or a language spoken between people as a middle ground effort between both their native languages and easily accessible - which I think will not happen to Chinese as we know it. It could happen to a different version of Chinese though! Maybe Pinyin could be the next world language, but it's not exactly a language per se.
For anyone interested, David Crystal has written many interesting books on the topic of English as a world language or World Englishes.
I don't think that Chinese will not become a world language either. At least, it won't become one any time soon. But I believe that Chinese is somewhat a lingua franca already. In places like Malaysia, Singapore, Indonesia, China, etc. Chinese has become a middle ground language. Only secondary to English but it is catching up fast.
Ja lično mislim da je kineski još uvek malo dalje od toga da postane svetski jezik,pogotovo zbog svog pisma.