We need a healthy earth to sustain life for, now and future. The animal population is controlled by predators and survival of the fittest.
But there is a problem with human population. Earth to be healthy can sustain only one billion, but now it is seven billion, and growing.
Spreading of democracies, diplomacy between countries, organizations like U.N. led to lesser wars. Further, countries becoming large and powerful with more and more deadly arsenal like nuclear bombs understands the danger of war. It may wipe out large populations and contaminate earth making life unsustainable.
On top of this advances of science and technologies improved health reducing mortality and increasing life expectancy.
Strangely all good things and values we appreciate is not good for earth's health and goes against sustenance of life.
Unfortunately the concepts like populating space that was in 1960s didn't materialize. It is very evident that even populating oceans and seas by making under water worlds is not a reality. All these advancements in science to utilise currently uninhabitable places have failed, even if possible can be done only in small scale due to costs involved.
The world understood economics much later then pure sciences. People now realise that certain things cannot be done due to costs involved. Actually certain advancements in science stalled due to costs. Definitely space exploration is one. Exploration diverted in directions where profits can be earned or at least targeted. The value of money is mostly responsible to these changes.
However, no-one still realise only by birth control we can overcome these difficulties. We should not live on dreams, we must consider the reality. It should be appreciated the policies adopted by China to control population.