I don't like the UN, but its the message, not the messanger that should be heard here. They are accurately pointing out the consequences of low population growth in DEVELOPED (read western) nations to the stability of the economies of those nations. Most of Europe, China, and other places are in slowing growth or population decline and the impacts on work force availability and retirement costs pointed out here are correct. The US would be in the same boat if it were not for its immigration policy, like it or not (and I am not in favor of open borders - I do favor the 'Bring us your best and brightest who want to assimilate' policy).
OTOH, the poorest nations, and coincidently the muslem populations, are growing while their standard of living does not improve. The result is an unstable future and threat for us (those western nations and civilized governments.)
This warning is echoed in studies from our own think tanks and advisors on national security, so focus on the message.
You do have my permission to shoot the messenger. :)
It is not so much the stability of the economy that is threatened.
It is more the wealth-producing methods of the wealthy that are threatened.
Wealth-producing methods that depend on the Sisypean need for increasing numbers of people to sustain production.