Aunts and Uncles (genetic or not) are of huge benefit to children. maybe you don’t have kids. But if your best friend has a kid and you become Uncle Meow for that kid, you’ve just increased the security and stability of that kid’s life. You now are another adult in their life who can protect that kid from social and financial disasters.

Maybe the way to look at it is “we all have fewer kids, and invest ourselves in the welfare of the kids around us.”