Too many people on the planet. Global warming, the myriad problems caused by global agriculture, marine fisheries collapses worldwide, and now this shit. Too. Many. People.
Lots of possibilities out there!
Do we limit that to the young, or how else do you actually reduce the population in a meaningful way rapidly?
Your position should be controversial because it is incorrect. The ecosystem dynamics of humans are nothing like that of other animals.
It is possible that the evolution of a fungus that could degrade lignin is what brought about the end of the Carboniferous, with an associated extinction event: https://en.wikipedia.org/wiki/Carboniferous#Fungi
And, of course, probably the biggest one of the all, the Great Oxidation Event: https://en.wikipedia.org/wiki/Great_Oxidation_Event
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None of this is to downplay humanity's role in our current climate catastrophe. But we are the first of a long line of organisms that have the ability to do something about it, which is not nothing.
I hate to be so grim, but your position is, "technology has allowed humans to dominate the planet and push up our carry capacity. Externalities are building up as a result of those technologies and they make our enviroment less hospitable. We have yet figured out how to not do that, despite the problem being significantly more difficult now that we are locked in to supporting more people. But we will do it, for sure. "
Is the last part saying that because their are have nots we are fine? You completely ignore the "we don't know how to make basic stuff at the quantities needed in a sustainable way" bit. I don't think banking on the discovery of new technology and having enough coordination as a species to implement that tech in a timely enough fashion is so safe that doubting it should be controversial.
...In other words, the problem is the rate of externalities per person. By replacing those systems with sustainable ones, we win.
And we don't do genocide.
It is indeed possible and sustainable to stop population growth. The thing is, Capitalist economies in most countries are fueled by the pursuit of "growth" - not of population, but of profits/future-profit-streams. When capital has less possibilities for growth by expansion to new people (or, still today, new markets developing) - there is a tendency to for exploitation to deepen. So the lack of the "safety valve" will make existing social arrangements more tense, or even worse, on average. Of course - that's an issue even without getting to zero-growth, as new markets in developing countries are gradually running out.
(Haven't talked about controlled population reduction, to not run on too much in this comment.)