So far in the history of lingua francas, there was always a notable rise of the lingua franca until it reached a peak, went into decline and was replaced by another lingua franca. Of course, we can't imagine now that the USA and English will lose their influence but our forefathers also thought that the Roman and the Chinese imperial empires would last forever. A political or economic world war can very well change the power structure in the world and lead to a new lingua franca supported by the new world power.
There is a discussion between linguists going on about the role of English in the long-run and eventually, only time can tell. I'll like to quote two interesting articles.
"... And always there is the resentment generated by dependence on a language which has to be learned, and therefore favours elites over those without access to schooling. Prestigious lingua francas are socially divisive, and therefore unstable.
English in the global age is often portrayed as an exceptional case. Writers who take this view point out that English differs from previous lingua francas in two important ways: first, it has no serious competition, and second, although it was originally spread by conquest, commerce and missionaries, its influence no longer depends on coercion. Because of this, the argument runs, it will not suffer the fate of its predecessors."<o:p></o:p>
(to be continued)
I think English will stay the lingua franca forever. Certainly empires come and go, but English has made too much head start now, especially in computing, internet, science, business and law. Chinese, its nearest rival, has too complicated a script.
As @Miriam says, coercion drove French, etc. and English. Some former colonies have replaced their French with English, others have kept their English and made it their own. These days English is not forced but willingly learned.
"Indeed, we disagree with the idea that English will be “The Last Lingua Franca” not only because it assumes that technology will make lingua francas obsolete (which is how Ostler intends the phrase to be interpreted) but also because we see no reason to suppose that English will remain the dominant global language forever.
At present, as our various studies have shown, it is difficult to identify any individual pretenders to English’s title. By sheer numbers of speakers, Chinese, Spanish and Hindi / Urdu would seem obvious candidates, but examination of these languages’ influence or their positions in global language networks shows that they have a lot of ground to make up before they can be seen as realistic challengers. Furthermore, there is no evidence at the moment that users of these or any other languages have any inclination to unseat English as the world’s lingua franca."
<o:p></o:p>(to be continued)
Here it might be objected that Ostler's argument depends on an unrealistic techno-optimism, and puts too much emphasis on the supposed primeval bond between speakers and their mother tongues, which some would say is largely an invention of 19th-century European nationalism. But even if he is wrong to predict the return of Babel, I do not think he is wrong to argue that English's position as the premier medium of global exchange will not be maintained for ever. In the future as in the past, linguistic landscapes can be expected to change in line with political and economic realities. "