The problems of the world have so little to do with the numbers inhabiting it, it is more about the unequal distribution of resources, the way capitalism and corporatism funnel profit towards a very few, power, greed, all dressed up in a superficial lip service to "the greater good".
It's not a case of either/or, and I get a bit frustrated that everyone who cares about the problem of distribution and concentration of power also insists that there is no overpopulation.
The global birth rate may be falling, but our population has grown so much from 2 billion to 7.5 billion over the last hundred years that it can keep growing when people have fewer children.
I remember reading a breathless article about an amazingly large population of seals -- the biggest population of large mammals excepting humans, their pets, and the billions of animals we breed to slaughter and eat. This excitingly huge population of seals was six million. There are 7.5 thousand million humans. We really do exist in disproportionate numbers.