Humans are already past the sustainable limit. Now we may come up with new technologies that make the current population sustainable. The Haber-Bosch process allows the Earth we support 2 billion more people than it would otherwise. It would take 6 Earths to sustain everyone at American style standard of living. The future is hard to predict. We have some pretty good ideas of what is likely to happen but how it unfolds may be very different than how we imagined it. With climate change overuse of resources and reduction in biodiversity humanity is potentially heading towards a disastrous collapse. However we have a lot of bright people trying to prevent that from happening. And if food becomes scare, well, we'll always have Soylent Green.