newman: "...Now the earth is overpopulated. The mitzvah was fulfilled by about 100 years ago..."
A mitzvah is not a "suggestion"...it is a commandment.
We are commanded to obey, and if we truly believe that G-d spoke to us at Sinai, then we are obligated to obey G-d and not the scientists at the UN population control committee.
If mankind obeyed the dictates of our Creator, and stopped hating and warring against each other, the resources and creativity of mankind would be put towards feeding, clothing, sheltering and healing each other.
Another aspect of human reproduction as a Divine Mitzvah is that we must as human beings and as Jews come to an acceptance of the possibility that "overpopulation" and the famine, wars, and unrest that such a situation can produce, might indeed be part and parcel of G-d's Plan for Creation...just as we accept that tornadoes, hurricanes, earthquakes, and floods are natural occurrences and therefore must exist for a reason and purpose.