In 2040, the world will have changed dramatically. We will know that we're running out of various resources and the higher global temperature will be causing droughts, floods, and other phenomena frequently.
Because we are aware of this fact, we will be living more eco-friendly, will have found ways to substitute resources, and will be using them more effectively. The climate crisis will be the world's top priority and our politicians are willing to take steps towards a better future at a much higher pace.
People will actually want to have children again as they know that the world they are born in is a safe and good place. The highest good won't be profit anymore but protecting the beauty of our world.