Population - again

The issue has come again in my conversation, and I'd like to expatiate again.

The planet has a carrying capacity that cannot be exceeded. This is an undisputable point. The biosphere is a finite system, whether we like / ignore the fact or not.

We can only find out what that carrying capacity is, I suggest, empirically, by increasing the population. But we still have the problem of quantifying that carrying capacity. It has to be sustainable over time; if we sustained x billion for a month but then people started to die, would that have achieved our target?

Given that we know there is an upper limit, even if we can't quantify it, do we put the brakes on now? My opponent says it is better never to exist than to be born into starvation and curable disease, and that living on another planet is purely fantasy. But this involves speculation that the human population can't be sustained by technology as yet undiscovered and uncovers profound questions about the unborn which are perhaps the epitome of speculation.

If we should put the brakes on now, the problem I still have is with targetting those social groups that currently have the most children per woman. The idea that they can't afford them, or that someone else shouldn't have to bear the expense of them is really abhorrent to me. Is there another species that goes in for this kind of social engineering? Survival of the fittest may seem cruel intellectually, but it is unavoidable in practice.

Social engineering is aan abstraction. It is us imposing our will/morals on others when there are other options.