Now in the countryside the dialects persisted until WWI and I expect that to be the same in France's new possessions, English will be a minor language in North America or England, a peasant language, and French will be the prestige language.
This sounds all too strangely familiar. Wasn't this the situation a millennium ago?
You assume that the advent of modern communication will tip the balance in favor of French, but (making the assumption that modern communications technology arises in this TL, and that the French take advantage of it) by that point English (which will remain a language of some prestige, unless the French engage in a scorched earth campaign, burning all of the books and reducing all anglophone schools and universities to the ground) will be able to avail itself of this technology as well, as many languages have. If the French are ruthless, that won't make Anglophones abandon their language more quickly, especially if they have numeric superiority.
In order to suppress English, the French will have to do two things - swamp newly-conquered Anglophone colonies with French speakers, so that the Anglophones become a minority, and make French an attractive alternative (by using incentives such as universal education, the media, and so on, while suppressing the use of English in these realms). That's a tall order, and will take generations to accomplish, if it is possible at all. If the French have a globe-spanning empire, how many colonists will they be able to spare to exterminate English?
I disagree, it would be sadly all too easy to wipe out any number of languages. There are what, 2000 language currently threatened with extinction worldwide?
More, but you didn't read my prerequisites for language death above. Apart those examples that are the result of genocide, language death is
very slow. It almost always results after a prolonged period of stable or semi-stable bilingualism, as the moribund language retreats from spheres of daily life. Government and commerce are almost always the first to go, and these are the spheres that show the most intrusion in the form of loanwords even when the language isn't dying out. Religion and family life are inevitably the last to go, and these spheres usually retain a more pristine vocabulary. The language may even be reduced to the point where it is only used in one sphere (e.g. religion or scholarship for languages like Hebrew and Latin).
Finally, almost all of the languages that are moribund today are unwritten and undocumented (there are a few exceptions, under a dozen, which are the only ones that non-linguists seem to care about). When a language lacks a literary tradition, it must passed down from generation to generation orally, which diminishes its prestige and its utility to future generations. Thus the temptation for children to switch is so much greater. This temptation is greatly reduced if a language has a literary tradition (which is, in itself, prestigious - only 70 or so languages have developed a literary tradition, out of the thousands that have been identified) and can be used in multiple spheres of life.