Dystopian world where people aren't allowed to have kids, just because they are poor?
Much better is to make sure poors have enough money to raise their kids. Offer them government assistance.
One person would still need 8 litres of water, a piece of land, food and other essential limited resources.
How do you plan to let all the people have kids with an existing population density of 828 persons per km² without ruining it for everyone?
What other filter do you propose is realistic? Do you think there should be a test, higher age requirement, education background or something?
Clearly, the current population is too much here. Sure it is declining (predicted to be normal replacement fertility rate by the end of 2020) but that will take long time to materialize and then you have government doing something to stop it because failing economy. Why even set your whole structure in a way that requires numbers? This is a perverted incentive and should be stopped.
https://link.springer.com/article/10.1007/s10668-015-9711-5
Optimum world population is something around 2 billion. Not 8 billion and growing.
You cannot speak of an optimum without having decided a target. It's optimum everyone having their own private jet?
What a simple solution but of course, that won't happen because you need low tier workers in the society that the state and others can leech off from.
Rich need their prostitutes, drivers, maids, low cost labor and employees. Society need numbers, labor which they can abuse for cleaning the filth and boosting themselves in a sacrificial ceremony.
For example, UP state in india has a population density of 828 persons per km²
What good reason do you think there is not in spending money to stop poor people from having kids?
I don't get the ethical issue about it other than it being made up by people for patting their backs. Government can ban smoking, drugs, porn, video games, internet sites, alcohol, and many other things to keep you safe from "harming" yourself. Doctors can veto your opinion or voice if they think you are a danger to yourself.
All research shows kids living in extreme poverty has a dire future, lot of bad health issues and will most likely remain in poverty. Why can't we ban or at least try to spend money to that cause?
3.6k kids die due to hunger/deficiency every day in india. What if they weren't born?
Why have so many people that you need to resort to filtering for everything in life?