Mandarin will not become the next world language because English is politically neutral and is used at varying levels in several countries. On the other hand, Mandarin is mainly associated with China. No one wants to learn a language connected to a regime that heavily censors their media.
The PEW Research Center’s Global Attitudes Project surveys found that people in the Philippines, Vietnam, Korea, and Japan are more comfortable with the U.S. rather than China as a world superpower.
English will remain the global language because of its association with international affairs already. For example, English is the only official language of the Association of Southeast Asian Nations (ASEAN), not Mandarin.
Some countries resist learning Mandarin. For example, the Vietnamese government does not want to emphasize Mandarin learning in Vietnam. Vietnam and China are in conflict over disputed South China Sea area. Young Vietnamese people would rather learn English over Mandarin.
English is a language associated with free speech and free expression. English’s widespread usage allows ideas to be easily spread in English as opposed to Mandarin. 
Mandarin, on the other hand, is closely associated with China, whose tightly controlled media and heavy censorship do not have associations with free speech. English's widespread usage and association with freedom will ensure its continued use for years to come.