Problems arise when knowledge exceeds wisdom. We've come very far in our technological know-how in the past 300 years, but we're still in the Dark Ages, spiritually. Which means that we're essentially a nuclear-armed Medieval fiefdom. That's not encouraging for the longevity of our planet or our civilization.
Re: ecology, there are two schools of thought. One believes essentially that the Earth is round, a sphere, and therefore of finite size. It follows that at a certain point, it is no longer advantageous to convert Earth matter into human tissue, and that it's good to have X number of hectares per person for "ecological carrying capacity" or "ecological footprint". The other school believes essentially that the Earth is flat, an infinite plane with infinite resources, and that if we just keep making more babies and more bombs, then God will provide. I don't feel that providence was intended to pull us through any conceivable or habitual lack of planning or foresight, but I suppose that it feels like warm fuzzies to those who believe that God, the Force or the Source helps those who don't help themselves. I wonder what God thinks about that. I'm not sure I have much to say to the latter school of thought on this ... if we cannot agree that the Earth is finite and therefore requires a certain foresight once human civilization reaches epic, Earth shattering proportions (which it has, we're now an ecological force like erosion or the carbon cycle), then we're not living in the same reality.
The Axemaker's Gift (Burke) is an excellent book to read regarding the history of civilization and the increasing problem with what he calls "pathological intensity". Pathological intensity and centripetal control go hand-in-hand, and they tend to increase with population pressure. It's easy to believe that the Earth is not crowded when one lives in the American West or Midwest (at least for the next few years, before the Chinese buy it) ... but living in Bangladesh or Lagos, Nigeria for a season should inform that belief.
Sustainability involves a delicate dance with Nature. We must not forget that civilization depends on Nature, not the other way around. Nature is what we derive everything from as a civilization, and it's generally agreed among the majority (but of course, not everyone; we're a barbarian planet with lots of various and sometimes dissonant perspectives) that our civilization began to take from Nature more than it gives back at some point in the past century.
If we could become sustainable and subsequently figure out how to give back more to Nature, then, under those far-off conditions, perhaps it would be advantageous to further swell our population. If we ever decide, for instance, that sheer processing power or resource exploitation and expansion are more important than enjoying peace, freedom and environmental accountability/sustainability. At first blush, I'm not sure why they would be.
According to the general consensus of top biologists and ecologists who have been actively publishing rigorous peer-reviewed papers for the past 40-50 years, it's estimated that the Earth can indefinitely support approximately 2 billion people with a standard of living a little less than the average French person today. The human population of Earth is between 6 and 7 billion now, and China and India are just starting to fire up their industrial production centers and consumptive habits.
A famous Chinese curse goes, "May you live in interesting times."