Even if that was true, the people who are suffering the most in those high birth rate countries have very little they can do to alleviate that suffering. If there are major changes that could be made in a political system to drastically improve it, and they haven't been made, that is a sign that we are very far from utopia, once we ask ourselves "why hasn't that happened yet"? Either it's not that easy or simple, or it is, and the people with the capability to make those changes are unwilling. The former invalidates one of the assumptions behind that reasoning, and the latter tells us that rather severe injustices are occurring.

> high birth rates and could reduce poverty straightforwardly.

If humans produce more than they consume over their life, then reducing the birthrate would increase poverty. Your suggestion implies that additional humans are worth less than nothing, but that doesn't seem to be true, at least not in the many countries that have both growing populations and economies. Is there some country that's getting both bigger and poorer because its people can't support themselves?

> Many poor countries have high birth rates and could reduce poverty straightforwardly.

This is pretty reductive if not ignorant.