I live my life not for enjoyment but to try and leave the world better than I found it. If I can succeed in doing so (even slightly) then teaching 2 children to do the same would be my life’s greatest contribution.
I actually think the declining birth rate is rooted in the narrative that humans are ruining the earth. I believe we are making it better and more interesting over time.
I think this is unconvincing. People may cite this in a survey, but this is not how the vast, vast majority of people make a decision like this one. In my opinion, modernity has been increasing quality of life in developed nations for a long time, and recently it's begun a downturn - nothing can increase forever. It's not dramatic, but it is enough to cause a strong conflict with expectations, and one of the first things to go is the giving of your life to children.
If these things succeed in delaying child creation then you end up like me (if you are lucky) and pop out two kids in your late 30's with many miscarriages and discovering the third probably isnt happening with very expensive ivf and even then it is a crapshoot. If you are unlucky you find out you waited too long and indadvertently optedout of the gene pool, at which point the environmental thing is a c(nvenient excuse to use when asked.
There's also the dramatic cultural shift toward self-centeredness and self-fulfillment. "Follow your heart!" "Me time." "Self-care." In terms of caring for one's mental and physical health, this used to be framed such that you prepared yourself to be a useful and productive member of your family and society, not that you were doing it for yourself. The "follow your heart" nonsense emphasizes feeling over thinking, and seems to be aimed at much more, but also weighs in here.
Humans have been ruining the earth. But we need to raise better humans to fix it.