I don't buy it. Not one bit. The idea that there are more people living on Earth than can be sustained by Earth's resources is ridiculous.
If you have a glass that can hold 200ml of water, how do you fit 300ml of water into it? You don't.
If you fill it up to the top, it will overflow... The glass' capacity can simply not be exceeded. Any additional water you try to pour in will simply end up on the table, or the floor.
The Earth has a finite amount of chemical resources that can be used to support human life. Once those resources are spent, additional human lives will not be possible: pregnant woemn who do not have sufficient nutrition will be unable to carry their children to term... There will simply not be enough chemical matter from which to construct a new human body.
There CANNOT be more people alive in the world than Earth's resources can sustain. It's simple physics!
This is a separate issue from that of quality of life for those in poverty-stricken areas. There is no excuse for some people to live in the lap of luxury while others live on the brink of starvation. But that doesn't diminish the fact that no human being can live beyond the brink for longer than a few days.
Overpopulation is a myth.