6 billion people will die. Heck, 7.5 billion will die. Everyone dies. The question is not when, but how, and how do they live their lives.
I think we should pursue a solution not of genocide, but one of mass sterilization. We should choose about 100 million people from all over the Earth that will represent the genome of the next generation of humanity and sterilize everyone else. And contrary to the closet racists' wet dream, this should be done in exactly the opposite way: we should strive to maximize genetic diversity and make sure that everyone has a pretty close relative represented. It would have to be massively automated, driven by genetic data, and highly, highly scientific.
That's the only non-morally-horrendous depopulation solution that I can think of. It would solve our population problem in ~60-70 years, naturally. Humanely I would say.
To some, it might be liberating even. The need to not ever worry about supporting children, or really, about the future at all. They don't have to live with a lower standard of living (by much). They don't have to figure out how to compete with 8, 9, 10 billion other people trying to claw their way up to the western standard of living. The childless could focus on bringing meaning to their lives in other ways, can invest in the next generation in other ways, and can transcend their biological destiny, which seems to be a cruel joke programmed into us from the very first bacterium.
It'd be like creating a living ark within humanity.
And the resources left over, designed to support 8 billion, need only support 100 million. There'd be a plethora of energy and real estate. A lot of the Earth could be turned back to wilderness. And humanity would take a smaller role. By choice. Instead of it being forced on us by the system collapsing on our heads.