Calling it a tremendous risk doesn't seem warranted given that it will not produce a significant drop in the earth's total population this century (see my comment above). A top-heavier age spread may strain political and economic systems: these are the social effects you mention. But from an ecological standpoint, not much will change for some time.
There is tremendous risk in covering the planet with ourselves and our waste, though; imho the more relevant risk, especially since we're pretty much locked into the pattern now.
If you look at it from much longer term perspective, it's scary because it's happening extremely fast (1 generation) within nations that have reached a certain level of material prosperity and stability.
It's also 'scary' because it's never happened before.
'100 years' when we will really see the effects more globally, is not a 'long time', it's a 'quite a short time'.
We only ever knew 'one natural direction' and though it's exploded since the industrial revolution, it was at least, in normal times, 'up' to the extent the system could support it.
So yes, it's not 'scary' right now, and as you point out, Mother Earth could use a break from all of us. But thinking about the next 'few hundred' years, it's spooky.
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.
I would agree but for the fact that if we look at habitability prospects from that same longer-term perspective, we see an ocean of scarier things happening just as fast.
Humans have endured and are generally resilient to population fluctuations. We and our industrial societies are not resilient to e.g., significant departure from the Holocene equilibrium[0].
There is tremendous risk in covering the planet with ourselves and our waste, though; imho the more relevant risk, especially since we're pretty much locked into the pattern now.