In What We Owe the Future, William MacAskill asks such questions as “Is It Good to Make Happy People?” In his evaluations, he simplifies the analysis by ignoring the impact that any additional human might have on other humans (and implicitly, that they might have on animals or the earth).
I would propose that it would be far more accurate and useful to do such an analysis if we were to treat the earth as a living being, with all life-forms as cells of that being, just as we are living beings composed of many cells. This is a way of describing an ecological perspective that has the benefit of giving us an intuitive view of the situation.
Human beings could be considered as comparable to the brain cells of the earth. Given that, clearly the extinction of humanity would be an enormous loss. On the other hand, humans are leading to the extinction of a great many species on the planet, in part by crowding out habitat. In fact, it is not a stretch to consider humanity to be an invasive species. From this perspective, the idea that adding more humans creates an overall good might well be misguided.
Just as the idea that GDP can grow indefinitely is clearly absurd, since planetary resources are finite, so the idea that increasing the number of humans is better than having fewer is clearly true only up to a point, after which the negative consequences of having more humans than the carrying capacity of the earth outweighs any benefits those additional humans might bring. This does not preclude humanity from expanding by colonizing other planets—many plants send out seeds in order to increase their presence, rather than simply growing to enormous and unsustainable size. But on the earth, there is some limit to how many humans can live well, especially taking into account the well-being of all life on earth. I do not claim to know what this number is, or whether we have surpassed it already. While it appears that we have, since we are badly fouling our own nest, it might be possible for us to learn to live more in harmony with nature so as not to emit excess greenhouse gasses, not to destroy habitat for our fellow inhabitants, and perhaps even to get along with one another.
In other arenas, MacAskill is clear that (for example) additional dollars may provide diminishing returns when contributed to improve a particular form of human suffering, he has not expressed the same insight regarding additional humans. We modern humans tend to carry the perspective that each human is separate, independent, and unrelated to others when considering their value, rather than recognizing (as many indigenous people seem to) that we are all deeply interconnected and interdependent with one another, other living beings, and the earth. I do note that MacAskill and many other people involved in EA do at times express concern for animal well-being, but this does not seem to go as far as a full ecological perspective.
I might point out that this idea about the earth being a living being has actually been believed by many people in different cultures and times. Wikipedia says, “Although the concept of the anima mundi [world soul] originated in classical antiquity, similar ideas can be found in the thoughts of later European philosophers such as those of Baruch Spinoza, Gottfried Leibniz, Immanuel Kant Friedrich Schelling, and Georg W.F. Hegel (particularly in his concept of Weltgeist)”. It further points out connections with Platonism, Gnosticism, Hermeticism, Jewish mysticism, eastern philosophy and more. In modern times, spiritual ecology and the idea of Interbeing popularized by Thich Nhat Hanh express this same concept.
But whether accepted as describing something actual or just as a useful analogy, I would propose that taking this ecological perspective is far more useful in evaluating moral good that acting as if human beings are in some way independent of, and divorced from, the earth and all other living beings.