The article I wrote covers (to my satisfaction) the reasons why the overpopulation arguments are fraught, and often just plain wrong, but another thing you might like to consider is that, lack of contraception aside, human history has built its ideas of childhood on how having children benefits the parents. For example, in many cultures (old agrarian, but also many modern developing cultures) having children is based a lot on spawning workers who will look after parents in their older age. Equally, even in the
most couples who plan to have children have them for the benefits they will
bring to their lives (the fact that a new life is created with its own unique
life is a great and special factor too). UK
The fact that the world continues to become more developed and prosperous, coupled with the fact that more and more people are living in big cities, means that many of the factors that make overpopulated areas ill-equipped to deal with it are becoming less and less of a problem. The more it's the case that parents choose to have children on the basis of a rational cost-benefit analysis, instead of needing children to help survive old age or women not having proper control over their reproductive cycle, the sooner population numbers will begin to more closely resemble science's law of parsimony.