If I could place a wager, I would say this will not happen. I fully expect some kind of cataclysmic plague to sweep the world. We use way too much medicine (antibiotics), no one knows how to fend for themselves and people travel the globe in a day.
I think all these factors and more I haven't mentioned or thought of will combine to wipe out a significant portion of the population in the next 50 years.
And they are telling us this because it is the fault of the US, and we will have to account and provide for these people? Because naturally, if we don't, (but thruthfully, even if we do) they will hate us and want to cause us harm.
I say stop the gravy train and let them fend for themselves. Even animals are smarter than that. Animals reproduce at a rate that their environment can sustain.
The US can't be the cure all for the Worlds (and the UN's) continued stupidity.