Depopulation is a disaster because in democracies, power is proportional to your numbers. If you don't have kids and your nation is invaded by millions of third world immigrants who want your money, your land, they have it.
Look at California, over 50% of newborn babies there are hispanic, but they're still considered a minority and given preferential access to education, contracts, jobs, etc.
If the depopulation was even, it wouldn't be a problem, but we're witnessing massive depopulation in the industrialized western world coinciding with massive population booms in the third world (made possible by the largesse of our dying civilization). In a sense, we are financing our own extinction.