Nothing "natural" would sustain 7b+ people anyway, so what's your point against "sustainable" as a reasonable (and considerable) improvement?
"Sustainable" is just a label, it doesn't in itself prove
a) that the methods it describes are sustainable;
b) that the conventional, industrial methods are not sustainable.
However, I agree that nothing natural can sustain a population of 7 billion. But we can try to concentrate the necessary unnatural food production on the smallest possible area, or use less technology and spread it over a much wider area.
This is largely an overconsumption problem, not an overpopulation problem. Most of the people born today will emit less than you or I ever will.
It's true, and it's enraging. But at the same time, are you wishing for the people in the developing world to keep producing so few emissions per capita? Because they would emit as much as we do if they could, and I'm sure they dream they'll be able to do it one day. And it's fair: but then we need to think of a recipe for sustainability that factors in much higher consumption levels for the now developing countries- instead of much lower ones for the developed world.
I don't want to stop doing any of these things, they make my life great. I could radically change my entire lifestyle to something much more limited and uncomfortable, only for the gains to be eaten up because my neighbor refuses to use contraception, and refuses to teach his 5 kids to use contraception.
The onus of climate change falls on people whose carbon footprint is exponential. Not me, mine is finite. I'm not going to move mountains in my life to make a difference that's fractional compared to other people's careless reproduction.