But, if humanity stays below replacement rate, humanity goes extinct.

I don’t think this is a real risk. And if it were, it certainly won’t be anytime soon. Fewer people means fewer mouths to feed, fewer homes to build/maintain and less consumption in general, which given how the planet is struggling to continue balance with current human resource consumption, a gradual decline in human population would probably be beneficial in the long run.

To actually threaten humanity’s continued existence the number of humans would need to dwindle so low that the societial and the medical infrastructure that permits/causes the declining birth rates would completely collapse and people would naturally start having more kids again in order to keep up with the work on the farms that most people would need to work on at that scale of society

Edit: Put more susinctly, the current declining birth rates are a because of societal changes, not biological ones

My wife has one branch of her family where 4 generations occured in a 60 year period, the 5 generation photos are pretty neat, and also I’m really happy for the last generation which broke the cycle.

I knew someone who was by her own description baby-crazy as a teen and basically seeked out someone to knock her up. Hormones are wild things sometimes. Her child did not repeat the cycle fortunately