Fully agree Bob. When you look at a human population graph over time, there was more like 200 million people sustained for a very long time without the aid of technology.
http://www.e3ne.org/wp-content/uploads/2015/04/JCurve5_2014.png
I suspect sub 500 million is more realistic for an environmentally sustainable population that can last for millenia. And that was when the environment wasn't completely trashed in so many ways.