That's a good point, Marcus. Perhaps the tones of spoken Chinese really aren't more difficult to learn than the consonant and vowel distinctions of English -- I'm not sure, because I've never tried to learn Chinese!
I still think that the complexity of written Chinese presents a barrier, but if China decided to promote international learning of spoken Mandarin written in some form of pinyin romanization, then I imagine it would be possible for Chinese to replace English as a global lingua franca.