Even once the developing countries reduce fertility below replacement (which will not happen for a long time yet) with increasing life spans and improving medicine the world's population will still continue growing(slow growth is still projected in the year 2100), then stagnate and then one day very far from today slowly decline...

but it will still take centuries to reach the population levels of the early 20th Century (only reached 2 billion people in 1927). In the meanwhile we have too many, not too few, people around.