Because we're anxious about our investments in soylent green.

They're concerned about "their" people, because it's declining only in rich countries and those tend to see themselves as "better" and don"t like "unregulated immigration" (while the regulated one costs shit tons of money). Also those who bring thst up are usually right-wingers.

Or to say it bluntly: Xenophobia and racism.

Because it means there will be a larger burden on (smaller) younger generations to look after the aging ones.

Instead of 6 kids and 30 grandkids, you maybe have 2 kids and 1 grandkid, if you're lucky.