I... Look, can we be real for a minute? None of these are why people aren't having kids.
Climate change is primarily affecting the global south, countries that have had the slowest drop in birth rates. Air quality globally has been increasing for decades. Yes, the current generation has less attainability than their parents, but modern life quality is still higher than any point in history. Even if you were to attribute it to that decrease, there have been decreases in resources between generations before without anywhere near the dropoff of births. Sumerian peasants didn't have this level of decrease during concurrent wars and famine, both demonstrably worse conditions than most people alive will ever face.
Current studies point to one correlative factor: Smartphones. Now, we can discuss why: Personally I believe it is a mix of several reasons, including beneficial ones. I would argue that smartphone have made people more aware than ever of the unfairness of life in modern society, as an example of a beneficial cause. 30 years ago, a woman in Nebraska wouldn't know intimately the firsthand account of a modern genocide, for example. Let me be clear: This is undoubtedly, unequivocally a good thing.
But we also need to address the sheer amount of slop content and unending access. Shortform content is proven to inflict users with symptoms reflective of ADHD and anxiety. Social media is built to make us not just dependent but addicted. The rise of smartphones has been part and parcel to the loss of third spaces; it's not just that people lost community, it's that we have also replaced it with instant access to online communities that cannot and will never be a viable replacement. We let the rich finally build the ultimate circus. And fixing that will not just require a fundamental restructuring of society, but also tough conversations about our responsibility to escape these systems.