But English is already known by most people in the world, unlike Mandarin Chinese. (And Mandarin is a hard language to learn...) It would take too much effort to dislodge English, and the Chinese won't make that effort (unless they have some kind of cultural revival, unlikely under the current management). Mandarin may contribute to a common pidgin, or everyone might end up bilingual.

Also, you're neglecting that unless there is a major shift in US foreign policy, the power structure will not change without a world war (or possibly some other massive disaster that renders the US powerless).

Doesn't look too good for us either way. But we'll see how all these predictions turn out.