There have never been past instances. We had a much higher fertility rate, then it just plummeted. It hit 2.1 and plunged below that moments later.
Increased lifespans will soon come to an end, but even if magically they could remain you'd just end up on a planet where everyone's too old to have children anymore, and human extinction is locked in. Every second fertility rate's below 2.1, there are fewer people of reproductive age and within your own lifetime there won't even be enough of those to sustain a world population in the tens of millions if suddenly they switched back to 2.1 (and they won't). This is doomsday. But be sure to have fewer children to reduce your carbon footprint.
Increased lifespans will soon come to an end, but even if magically they could remain you'd just end up on a planet where everyone's too old to have children anymore, and human extinction is locked in. Every second fertility rate's below 2.1, there are fewer people of reproductive age and within your own lifetime there won't even be enough of those to sustain a world population in the tens of millions if suddenly they switched back to 2.1 (and they won't). This is doomsday. But be sure to have fewer children to reduce your carbon footprint.