It seems that mankind is going to stumble into a state of disaster. Just as herds of animals can grow too large to be sustained by the land they live on and their numbers dwindle so it will be with the human species.
We will go on stuffing our faces with food, living for greed and money, burning up our precious resources for as long as it lasts (and we have the filthy lucre to pay for it) until it is too late. Famines, disease, war, financial collapse and of course death will be inevitable as the earth will pay us back for what we have plundered.
No a hunter-gatherer lifestyle is not the only sustainable lifestyle, farming in harmony with nature is also viable. Although I haven't done all the science to prove this point (who has?) I believe it is probably more sustainable.