Leveling off or lowering the human population through lower birth rates works better. I'm not saying it's simple or even possible since so many people jump to conclusions about eugenics etc, but reducing overpopulation makes a lot of problems easier. If we don't figure out how on our own, nature will do it for us so we might as well figure out how in a peaceful, fair way.
But this doesn't really solve the city/urban problem as the vast majority of economic growth occurs in cities and the continual migration of rural to urban happens even when total population is shrinking.
Lowering population alone doesn't solve every problem, but without it, many problems are unsolvable.
If you really want population control, you don't need eugenics - you need Logan's Run.
Consider, say, Iran. In 1980, its birthrate was around 6. Now, it's 1.4, lower than Europe, and lifespans shot up at the same time - but only for those born during the health revolution there. So those people are still less than 40 years old. Iran is just now starting to feel the impact of increased lifespans, and it'll take another 50 years for that to truly stabilize.