I think war caused by conflict for resources is a much more probably cause of societal collapse, and indeed extinction.

Lower numbers of people in the world reduces the chances of that.

Birth rates are much easier to reverse than climate change - it's a really odd thing to focus on.

Area's of the world where birth rate is still high tend to be in poorly developed regions where human labour is required to survive.

However, in developed countries - the idea that we need the same or more people to maintain civilisation when the means of production is being increasingly automated doesn't make sense.

As an example ~100 years ago in Europe you were looking at nearly 50% of the population being involved in food production, whereas now it's about 5% - and that includes a 50% increase in the population to feed.

Note the EU is a net food exporter - even if it does import bananas.

I was responding to 2 assertions, used to justify an argument, that clearly don't hold water.

ie a replacement rate of 1 isn't some immediate existential threat, and that making arguments like 'species aren't happy' is completely specious ( sic ).

If you want a wider argument - then the reality is the birth rate is dropping because when women have the choice ( via birth control and increased socio-economic independence ) they are largely choosing to have less children. So by definition, given it's a choice - women are happier.

The second bit is speculation on motive - which is why I didn't bother the first time - but in my view, those that push an argument that there is a fertility/species crisis ( using specious arguments like the above ) - who say women aren't really happy with their choice etc - is all about subjugating women to baby making machines to satisfy the ego of people like Musk. Whether the original poster has that motivation or has just fallen for the "species crisis" ( or even worse 'race' crisis ) rubbish - I have no idea - I didn't want to assume the worse.