How do you propose that we make the third world more sustainable? Our ability to 'sustain' [I put sustain in quotes because while we are able to sustain our population in the short term, our resource use is in no way sustainable long term] our current population was due to technology improvements provided by the mass production of oil products. I don't agree that the planet can sustain 11B, when that is 60% greater than our current population and we already have major sustainability issues. Not only will our energy production need to vastly increase, I have trouble seeing us able to sustain the necessary production of fresh water, food, and coarse grain sand (for cement), to provide the adequate basics for the population.
There is already shortages of all three of those basic necessities around the world. Cement is the 2nd most used substance in the world (behind water), and that is the basic building block for any modern structure. How do we house these people? Food production was greatly improved through the use of oil products (fertilizer, tractors, etc), are we going to continue to increase our oil use to attempt to produce enough food for 11B people? Where will the farm land come from? Not to mention, we already have major climate change problems, and increased oil use won't help that issue.
Water supplies are getting lower, we have cities like Las Vegas trying to change policy to use Lake Michigan as a supply of fresh water, where is all the water going to come from as the populations continue to grow? Desalination units? They're very energy demanding.
There is no stopping the population increase, that I agree with, and it will happen so long we can 'sustain' that growth, but is it truly long-term sustainable once we get there? I doubt it, considering our current lifestyle at 7B people isn't sustainable.
/edit: spelling