There are a couple of indicators that English is now at its apex as a world language. The proportion of people in the world speaking English as a first language is declining. One study shows that almost 10% fit this category in 1958. In 1992, this figure had fallen to under 8%. If this is a steady rate of decline, and one can assume it is, it should be under 7% by now. How can you assume this a steady rate of decline? Because the people who speak English as a first language are not breeding as fast as those who don't. They are, for the most part, members of the First World where low birthrates are now fairly common.
The other factor that will probably lead to the decline of English speakers in the world is the advent of translation programs combined with the increased use of computers. These programs are getting better and better, and it's not unlikely that within the next 10 years, there will be reliable translators which will actually do what they're designed to do. In fact, learning a second or third language might become a thing of the past altogether.
Right now, 76% of the content of the Internet is in English. The runners-up are Japanese, French, German and Chinese, in that order. This leads many to think that the future of English as a world language is inevitable. This may not be true at all.