Nicholas Ostler answers in The Last Lingua Franca: English Until the Return of Babel (2010):
The decline of English, when it begins, will not seem of great moment.
International English is a lingua franca, and by its nature, a lingua franca is a language of convenience. When it ceases to be convenient—however widespread it has been—it will be dropped, without ceremony, and with little emotion. People will just not get around learning it, not see the point, be glad to escape a previously compulsory subject at school. Only those who have a more intimate relation to it, its native speakers, may feel a sense of loss—much as French people do today when their language is passed over, or accorded no special respect. And those who are conscious of having made a serious investment to learn the language—having misread the signs of change afoot in global communication—may also feel cheated, even disappointed, when others seem to be excused from having to know it. But the world as a whole will shrug and go on transacting its business in whatever language, or combination of languages, next seems useful. (xv)
See more from this book.