As a health advocate, my primary purpose in suggesting a (voluntary) reduction in reproduction is to promote the wellbeing of present and prospective generations, rather than merely preserving the perpetuation of our species (though the two are intrinsically positively correlated).
Humanity’s greatest predicament is primarily propelled by overconsumption, to which emotionally and spiritually uncultivated people possess an inherent inclination. Human’s insatiable impulse to amass and acquire has devastated the planet (soil, water, air, food, biodiversity, etc.) and ruined our health.
Irresponsible/excessive resource extraction and wanton waste generation inexorably lead to ecological destruction and severe health repercussions. Unbridled procreation is bound to exacerbate the plight and amplify suffering, as posterity is condemned to a contaminated planet and a litany of chronic degenerative disorders.
The prudent thing to do is choose not to reproduce, or limit families to one child. Alas, such message is profoundly unpopular, partly because it directly conflicts with the perennial pursuit of endless economic expansion espoused by corporations and governments, whose priorities are profit and the longest possible postponement of the dire ramifications of unsustainable economic practices.
One should bear in mind that curtailing procreation is not a panacea for humanity’s problems (it is a well-known fact that a small portion of the populace consumes the vast majority of resources). It is only one—albeit an indispensable—part of the solution.
The Earth has the capacity to sustain a much larger population, provided it is predominantly composed of rational, emotionally mature, and spiritually evolved individuals–unlike the majority of its current inhabitants.