Traffic jams, pollution, loads of wastes, material consumption, needs to administrate all that, size of the cities, ability for justice to be served, hospital facilities, availability of pre-schools, number of children in class rooms...
People think overpopulation is a problem when you reach a certain cap, but I don't think there is a cap. I think it's progressive.
Plus with our current population size, we are bound to certain kind of systems and can't try alternatives. We have to normalize, we have to batch, we have to have pyramids, a chain of command to manage things.
This is great to produce and consume a great quantity of goods, but not so much for education, progress as a society, as a specie and even for something simpler such as producing goods of great quality.
But even if you disagree with any of that, and my whole population size theory, great birth control would solve one current major problem: unwanted pregnancy by men.
I'd say about half of my male friends became parents unwillingly for a lot of different reasons and scenarios. It does not make them great fathers because they didn't want to be one at this time of their life in the first place.
I think a global solution for a safe, non invasive and permanent procedure to keep one sterile at will would improve immensely the quality of life, eduction, and growth of children. Because the ones being here would be the ones wanted.
Which means basically shaping a better future.