Too far is an understatement.
People keep poking at the wrong reasons, but in some societies it is quite dire. South Korea with this year of 1, when 2.1 means 'static', means more than halving the population every 30 years or so.
For a reverse comparison, if you take a penny and double it every day, you end up with > $5M in 30 days. And yet this birthrate issue doesn't take into account plague, war, natural disasters, and potential issues with lack of food(starvation). And the worst of it?
Is that I believe it is 100% environmental.
People think "having children" is a conscious choice. And sure, there is some of that. But at the same time, it is the very point of existence for an organism. Actually producing children (not just performing the sex act) is an evolutionary requirement. It is literally the primary drive of existence. Risky behaviour is ingrained into us, if it enables the possibility of reproduction. The drives and energy we place into everything we do, has a background drive that is sexual in nature. We seek to excel, to impress the opposite sex.
Like it or not (I'm not like that, I decide, not my hormones!), this is effectively an accepted fact of animal psychology. It's a part of who we are, our culture is designed around it, and every aspect of our lives is ruled by it.
Why am I on about this??
Well, my point is that this is a primary drive, interlaced so deeply that it affects every aspect of who we are. Reproduction, the production and raising of offspring is an act we are, naturally, compelled to. Forced to. Need to do.
Unless of course specific chemicals, maybe microplastics or all of the "forever chemicals" in our blood, are blocking that process.
Again, people will chime in with the popular "But it's expensive". No. Just no. Nope! My point above is that this is primal drive. People have had children in the depression, on purpose. Historically people, even with contraceptives, have had children regardless.
If it's about money, why is the birth rate declining in countries with free daycare, universal health care, and immensely strong support for parents post birth? Mandated career protection for mothers, months and months of time off after birth all paid. Immense tax breaks making children almost a profitable enterprise. In fact, in some European countries, it is more affordable to have kids than at any time in human history... and the birth rate still declines. It's just not about money. It just is not.
Why I think this is immensely important, is because we aren't seeing a rate, but an ongoing declining rate. The rate isn't just the lowest in human history, but the rate continues to decline. It's not '1' for South Korea, it's 1 right now, and will be 0.5 eventually.
What happens when no one can have children?
I further ask this, because the entire future of the species is at risk. People get all "who cares about going on", but wars do happen, plagues do happen, and I assure you I'm happy to be here, regardless of what the survivors of the bubonic plague thought at the time. Yet if we see a plague that kills 1/2 the population, where does that leave this equation? And what happens if we see a war that kills mostly those of child bearing age? What then?
My secondary concern in all of this is, we have very specialized roles these days. There was a time where a person could be a "a physicist", yet now there are 1000s of sub-specialties in such fields. And not everyone in the population is capable of expanding science. Of discovering 'new'.
My thoughts here are that we require a certain base number of humans to continue to expand science. If we have 100M humans world wide, I do not believe we'll be capable of expanding our current knowledge base, instead, I think we'll regress. There simply will not be enough people intelligent in a way functional to, say, physics, to expand that field.
So if our population decreases too far, we may not be able to resolve issues with, say, forever chemicals. Or with microplastics. Our capacity to do research and resolve such issues may vanish.
Couple that with a graph that is constantly declining, and a simple 50% death rate in a plague, could mean the extinction of the human race.
So my real concern here is, we aren't swinging the pendulum on purpose. It's happening to us. We're in the middle of an extinction event.
And it's only going to get far, far worse.