There are many reasons why Mandarin will not become the next lingua franca, most importantly the writing system which unifies China but is even hard for Chinese to learn. As it looks now, it seems more likely that more and more Chinese will become fluent in English and that some English - possibly including a lot of Chinese loannwords - will be the most widespread language in the 100 years from now.
But I think you were downvoted for the wrong reasons, since otherwise the argument is fairly sound. Chinese influence is continually increasing world-wide. It's mainly the writing system that prevents a wider spreading of Mandarin, and the Chinese cannot give it up because it unifies their nation. Another possible scenario is that other languages of China die out even more quickly than they already do now, thus paving the way to full mutual comprehensibility via Mandarin, which would then allow a drastic reform of the writing system. However, this seems unlikely to occur within the next 100 years.
But I think you were downvoted for the wrong reasons, since otherwise the argument is fairly sound. Chinese influence is continually increasing world-wide. It's mainly the writing system that prevents a wider spreading of Mandarin, and the Chinese cannot give it up because it unifies their nation. Another possible scenario is that other languages of China die out even more quickly than they already do now, thus paving the way to full mutual comprehensibility via Mandarin, which would then allow a drastic reform of the writing system. However, this seems unlikely to occur within the next 100 years.