Language is a primary component of a culture. Outsourcing it to anyone is problematic at best.
This is why many countries like France have l'Académie francaise, an organization dedicated to promoting the old lingua Franca (French). Likewise, the Sapir-Whorf Hypothesis (not to be confused with Michael Dorn ) posits that language not only reflects the culture, but even shapes how the world is perceived. When Robert Greenburg suggests that the aria would only have developed in a vowel-rich Latin and the hymn in a consonantal German, he is merely extending the hypothesis.
I write these things to say that every non-English speaking culture has a vested interested in not seeing English become the dominant culture. Already, many countries complain about American "cultural imperialism" via Hollywood is every bit as insidious as its hegemonic role as "the world's policeman."
Indeed, the driving force behind Esperanto - designed as a world language - was that no one culture would be given such power over world culture. Its creator wrote:
"The place where I was born and spent my childhood gave direction to all my future struggles. In Białystok the inhabitants were divided into four distinct elements: Russians, Poles, Germans and Jews; each of these spoke their own language and looked on all the others as enemies. In such a town a sensitive nature feels more acutely than elsewhere the misery caused by language division and sees at every step that the diversity of languages is the first, or at least the most influential, basis for the separation of the human family into groups of enemies.
The result, of course, is what you would expect. Regimes everywhere thought the idea nonsense. Might it bring a kind of peace? Sure - because the realized it was the same kind of peace that arose from when your culture was completely killed off. Replacing your culture with the culture of no one group in particular has proved well nigh impossible on its own - imagine if you had to agree on adopting the culture of an enemy, and you see why it just doesn't happen.
As such, the direct answer to your question of "Who would oppose English as a world language?" is anyone who wishes to preserve their own, non-English-speaking culture. It is a pretty formidable lobby.
Put another way (and here I'm channelling @Chad, so please forgive me all)-
Russians will never accept English, French, or any language other then Russian as their national language. If put to a vote, it would fail miserably - there is no support. Same would hold true in China, India, France, and a host of other nations. Indeed, even in the Netherlands, where English is highly related and basically the language of business already, Dutch News could only muster 51% support for such a policy in an unscientific poll. Things would go south from there.
English is a powerful tool. Studies have shown that learning it brings about a 10% increase in wages for non-English speaking countries. That is an economic motivation for it, and its best hope of being adopted universally. Just don't expect anyone to like it.