All plausible theories I've heard for the cause of humanity's unprecedented, historically low birth rates are things that could not occur in less intelligent species. (Birth control, women's rights, hedonism enabled by modern technology, etc.)

> isn’t that catastrophising

Yes, I'm only putting that forward as the worst case scenario to make the point that this can't just be ignored. As I said in other comments, this almost certainly won't actually result in extinction because there are other corrective factors which would occur long before that, but none of those scenarios are particularly desirable either. (E.g. Civilizational collapse returning humanity to pre-industrical birth rates, global takeover by theocratic governments that ban birth control, etc.)

The only non-catastrophic corrective factors that sound plausible to me involve some kind of intentional collective action on our part that reverses the trend, which won't happen if everyone's attitude is the same as the root commenter's. (Granted, maybe there are other possible corrective factors I haven't thought of, but if so I'd like to discuss what those are rather than just have the problem dismissed with a hand wave.)

Neither the fall in birth rates nor its rise is intentional. I struggle to understand why people think a mega fauna of 7-8 billion people takes intentional decisions. An individual takes intentional decisions. Humanity … not so much I think.