Actually 6.6 billion is not only sustainable but the Earth could easily support even more. It is simply a matter of making better use of land and resources. Using efficient farming methods highly crowded nations like the Netherlands and Japan produce about half of their food and could produce more. Nations like Canada have great capacity to expand agricultural production as does Russia, the United States, Brazil, and so on. Doom and Gloom futurologists like Paul Erlich have been predicting the dire consequences of overpopulation since the 1960s. They have never gotten it right even once. In fact, far from heading for disaster, the world's living standard has actually increased during that time period even as the population has doubled.
I concede that the Earth's population cannot increase indefinitely, but given the recent decline in population growth rates the world no longer faces that problem. There will, of course have to be adjustments for the people of the world to have a decent living standard. For example, it might be necessary to require that the richest 10% of the world's population stop hogging more than 50% of its wealth, and it may also be necessary to convince humanity to stop devising ways of killing one another and put the resources devoted to the arms race to better use.
I concede that the Earth's population cannot increase indefinitely, but given the recent decline in population growth rates the world no longer faces that problem. There will, of course have to be adjustments for the people of the world to have a decent living standard. For example, it might be necessary to require that the richest 10% of the world's population stop hogging more than 50% of its wealth, and it may also be necessary to convince humanity to stop devising ways of killing one another and put the resources devoted to the arms race to better use.