Many countries in the world have transitioned from positive population growth to near-zero or negative population growth, with no genocide. So you're presenting

Many countries in the world have transitioned from positive population growth to near-zero or negative population growth, with no genocide. So you're presenting a straw-man (straw-Marvel-villain?) argument about population reduction. 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.)