The world can't sustain a population coverage similar to that of NYC or HK. Those population centers are supported by millions of acres of farmland that grow the food they eat. If there are enough people, then we don't run out of living space for them... but we do run out of farmland.

Secondly, population will peak because of the very factor I mentioned—people in industrial societies are too busy to have kids. However, one important fact about industrial societies is that they constantly seek to improve themselves and gain efficiency. More automation means more people out of work (or consumerism-for-the-sake-of-feeding-the-cycle, but I'm supposing we ignore that option.) Take this to its limit, and no one works (except on creative pursuits, but that won't be everyone.) What do you suppose the rest spend their time doing? I'm thinking population will start shooting straight back up in a post-industrial society. Not because each couple has more kids, but because there are more couples, well, coupling.

You haven't explained to me yet what's wrong with sterilization in-and-of-itself; you're using it like Americans tend to use the word "socialist," assuming that it's scary without explaining precisely why it's scary. Couples can still have kids under this scheme. Couples can still have just as many kids. It would just take, say, ten times as long for a couple to conceive (the regular period being a month), meaning that there would be a nine-month "fallow" period between pregnancies. Each couple could have the same number of kids, but half the children would end up in the following generational cohort, meaning that the yearly growth rate would be halved. Since conception is harder, some couples would give up after a smaller number of children, lowering the total produced. This is, in the end, a much more lax scheme than even China's one-child rule.

I can see where you are coming from in that developing counties need to have their oppression lifted in order to stop "investing in the future" with overproduction of children. However, consider this: if you do kill off the warlords, and ensure that all the infant mortality goes away, but leave behind a current population far exceeding their own supply of resources, what will happen to that generation, and the few proceeding from it, before they reach the point where the cultural tradition of overproducing young gives way to an industrial lifestyle? Assuming the resources go unequally distributed, won't those still in poverty try to take those resources by force, and become new warlords? War and anarchy will break out wherever population greatly exceeds resources, and there is no infrastructure (e.g. police) to hold down an oppressive government from forming.

To beat tyranny, the population must sharply drop, or the resources must greatly rise, along with the removal of the government, or the attractor won't be escaped. Food genetically modified to lower fertility does both. It might not be the best solution, but it's a solution, and unlike most others, it's one that's impossible to screw up. Contraceptives can be discarded for religious or cultural reasons, but food must be eaten. Educators' teachings can be ignored as misguided, but your body's egg or sperm count is a cold, hard fact. Other resources given as aid can be confiscated by a warlord for their value (OLPC laptops, anyone?), but what value does food that lowers your virility have to a warlord that could just as well eat something else?

I'm not suggesting there isn't a better, social solution. I'm just saying that as purely technical solutions go, this one is pretty damn air-tight.