I think it's better to ask why should population keep growing? We don't need it and it physically has to stop growing at some point.
It seems much better to focus on growing quality of life and quality of environment rather than growing number of people.
Because you can’t stop people from breeding apart from killing them, at which point you’ve become Hitler. That or tax and social disincentivization at which point you’ve become the Illuminati. That or a procreation suppression field at which point you’re the Combine.
(Godwin point reached very quickly...)
Take a theoretical world where population was drastically lower (or individual consumption is hugely slashed amongst the top consumers): We would drop within the safe boundary for all limits simultaneously, because the underlying multipliers for all of them are number of people and individual consumption.
[1]https://www.stockholmresilience.org/research/research-news/2...
Still slowing population growth would be the easiest gradual way of dealing with resource and environmental issues. It also helps to spur automation and innovation by reducing the amount of cheap, desperate labour available (which is not a fulfilling existence for the workers either).
It can also be tied into ensuring that children have a good upbringing - introduce a child licence with requirements for raising a child to ensure some stability and responsible parenting - e.g. mandatory parenting and nutrition classes, $10+k in savings, living space requirements, etc. - this would help reduce crime and poverty and improve everyone's lives.
Would you still argue against a higher population once we largely solve the emissions issues? Or is there a more fundamental reason you’re against a higher population?