The trouble is that people living in third world conditions aspire to the first world lifestyle. If a magic wand, or rapid economic progress made such possible, the net human impact on the environment would increase 12 fold. Nobody thinks this is possible in any kind of reasonable timeframe. There is simply not 12 times as much oil production, or 12 times as much copper available. There is no plausible way the world's food production can increase by a factor of 12, and no likelyhood that our ecosystems could survive a twelvefold increase in production of pollutants and industrial toxins.
Countries with third world populations that can get their act together are making agressive attempts to move into the first world economies, most notably China and India. Countries that can't are providing floods of desperate immigrants to the first world. The Chinese who gets a good job and buys a car and the central American immigrant who gets to Los Angeles both more or less immediately move up into first world type consumption patterns.
There is no chance that the present pattern can be long sustained and no chance that these environmental problems can continue unsolved. In the lifetime of today's children and young adults we will either figure out better ways to deal with them or will suffer the age old solutions of nature: war, plague, genocide, mass starvation, and civilizational collapse.
The most hopeful signs are the rapid decreases in fertility being seen in China and much of the rest of the world.
Today's young adults are probably the first generation of Americans to see a significant decline in their living standards compared to their parents. It will likely be more dramatic for those still children.