I think they have no real idea of what the absolute limit the earth can support is. If Codex Alimentarius passes and they successfully destroy the value of our food with stagnant technology, I doubt the earth can continue supporting 8 million people. However, if genetic engineering progresses to the point where we perfect it, and it is done to enhance people's health and not just quick profits, it may well be possible for the earth to support many more than 8 billion. Not to mention that the scientist that is successful in unlocking all 26 dimensions and creating more will unlock the potential for earth to support nearly infinite populations.
But that will never happen as long as regulators have their way. With the regulators, we will be down to a few million due to disease or poverty of their creation. It seems that the regulators' attitude is that humans have no right to even exist on this planet.