Numbers of those who choose not to have children - apparently increasing - must partly make up for those who do. One of my siblings (4 of us) and his wife chose not to, and another had just one. The other two of us had 2 each, so a population increase of 1.
My elder sister’s 2 have had 2 each, so no increase in the population, and it looks as if my 2 and their partners will be reproducing just 3 of the 4 of them.
I don’t suppose concern for the planet will be much of an influence on people who have a truly visceral longing for a baby (as some of us do) but those who aren’t really bothered either way, or actively don’t want the expense, huge commitment and general mess and disruption, will find ‘the planet’ a useful reason to give, instead of just saying they don’t want children. Since although it’s a perfectly valid choice, some people still think it’s odd or selfish.