Why are there people worried about low birth rates and population collapse? Isn’t the earth over populated and less people would be better for the environment?
So, economically speaking, we're really kind of focused on growth. We want more profits every year... Which means we need to sell more stuff... And a great way to do that is if we have more people to sell to. So a lot of the models we're working off of kind of assume constant population increase.
And this isn't just limited to private industry... Social Security, here in the US, assumes that you'll have more people paying into Social Security than are currently taking money out. More workers than retirees. And for that to continue to be true we need constant population growth.
So a lot of our economics start falling apart if population growth slows, stagnates, or declines.
Environmentally... At this point, at least, it's less about sheer numbers than it is about resource utilization.
Obviously every human being requires resources to live. They need food, clothing, shelter, etc. And there is some maximum capacity for our planet. Some point where all the resources are being used and we absolutely cannot support another person. But we aren't there yet.
Right now we've got extremely uneven and inefficient resource utilization. Like, right now my wife and I are living on 1.2 acres of land in a nice, big house... And elsewhere in the world we've got a family of six crammed into an apartment the size of our bedroom. That's obviously inefficient. I obviously don't need a house that big.
And it's not just housing... I'll eat a big, greasy cheeseburger for dinner just because it tastes good. If we were really worried about getting the most food out of our planet we wouldn't be eating beef with such abandon. Or spending time/money/electricity/resources on luxuries like video game consoles or whatever.
So while we're definitely not doing our environment any favors... And more people are definitely going to put more of a strain on it... It isn't so much that we need less people, as we need to use our resources more-intelligently and pay more attention to our environmental impact.