Yes, it is impossible to tell, for reasons my learnèd colleagues have touched on. Allow me to expand one angle.
As I was saying to Martin Silvertant just before (wat de neuk?), I predicted the death of Dutch in 200 years as a postgrad, when I found out that university courses were being lectured in English in the Netherlands. That’s how it begins: replacing a language with a more prestigious language in some domain of public discourse. Then more and more domains. Then the home.
I was challenged at the time by another student. “When you say that,” he pointed out, “you really just mean some time you can’t immediately predict, don’t you.”
Well, yeah. And screw you for being right.
Is there anything on the visible horizon that would displace English like that? The British Empire has set, and the Pax Americana may be starting to; but English has a comfortable niche for a while yet as a lingua franca, even without them. Could Mandarin displace it yet? Not impossible (and something can be worked out with the characters vs Pinyin); but then again, as Mao put it in Nixon in China,
Our armies do not go abroad. Why should we? We have all we need.
The next hegemon, if China proves to be that, might be just fine with English sticking around as a lingua franca.
In that case, the race to expire English will be between the Technological singularity, Environmental Catastrophe, and the natural death of languages. With mass literacy and the nation state, language change has been slowed down; I’d normally give it 500 years, but it’ll be likely longer. Which means it’s a race between the robot overlords and New York being under water.
… Sorry about that last bit. It’s Electoralgeddon, we’re all going a little crazy.