I tend to agree. Additional human population is both a blessing and a curse. It kind of depends on the capacity of society to produce net-positive individuals, those than through their work can produce more value than they consume.
The core issue though, as you glimpse, is not # of people, but rather long-term sustainability. With the caveat that we are not even close to be able to have a true circular economy, the aim is to get as close as possible to this equilibrium:
things_spent = things_produced
Which in turn can be broadly decomposed into
population * individual_consumption = resources * productivity
* So, one thing that helps is to reduce population. In that regard, what is happening in developed countries is desirable. We are reducing population without killing anyone of forcing anyone. Yes, it will be hard to adapt our economies to (we cannot rely on old people dying quickly or young people to come in droves after them). At some point, we need to produce the incentives to get population stable, neither increasing nor decreasing.
* Another is to reduce consumption, which is hard in most of the world since they aspire for more rather than less, but can certainly be done in some developed countries where food and goods are over-produced and mostly end in the garbage.
* Another is to increase productivity, which is as always a mix of investment in infrastructure, technology and education. This we should definitely do and be bold at, but we should not just take if for granted. I'm very skeptical of the idea that since technology has grown so fast and saved us in the past that it will *certainly* do so forever, just-in-time.
And this is a very simplified picture, without accounting even for immigration, war, redistribution, fairness, politics, etc. But in essence, many things need to happen if we want to prevent unnecessary suffering.