The transition from 2B to 7.8B was also kind of fast: 90 years. The exponential growth we've seen recently is unique in human existence. Before the last several centuries human population growth was much slower. In fact, it seems unlikely that population growth will become that slow again without some sort of disaster scenario.
So this is slightly misleading as well, because it's a geometric curve upwards.
The 'inflection point' you're really looking for is the dawn of the Industrial Revolution.
But at least in that capacity - we know what happened: we harnessed energy and could provide for the material wellbeing of huge numbers of people, which can have it's downsides, but it's not all bad. Better 'more' than risking plunging towards nothing.
The 'downward plunge' is happening for somewhat unknown reasons, and it's fast, and we don't know when it's going to stop.
If you put on the afterburners on an airplane, and it goes up faster than expected, that makes sense. But if the airplanes starts falling out of the sky for no reason ... scary.
People who don't want to have a bunch of kids, don't. Does it have to be more complicated than that? Unless everyone makes this decision (which seems unlikely to me although that could just be because I have ten nieces and nephews) it's basically a self-correcting "problem". People who don't reproduce are replaced in the next generation by the offspring of people who do reproduce.
