People are of course free to choose not to have children. The point however is that if it becomes a trend and more and more people of certain communities have fewer or no children, while other communities continue to have more and more children, the demographics across the world are likely to change dramatically in a few decades.
Over the past few decades there has been a trend in which people in richer countries/communities have tended to have fewer children, while those in poorer countries/communities continue to have more children (I don't think these communities are have more than they did decades ago, the key different being the reduced birth rate in more affluent communities).
But of course those poorer communities have tended to, and continue to have, lower life expectancy so that has always had a balancing effect in terms of overall demographics.
The key is what happens as poorer countries/communities become more affluent as I expect they wish to do. History suggests that birth rates will begin to drop off as affluence increases. But there is often a 'drag' - in the poorest countries often birth and death rates broadly match, and indeed similar for the richest countries. But as the poorest countries develop there is often a trend where the reduction in death rates runs ahead of the reduction in birth rates, resulting in significant population increase.
