I don't know the work in question, but the extremes of agriculture we have gone to aren't sustainable simply from a soil destruction standpoint. We may figure that problem out too, but just assuming our ingenuity will get us out of any predicament we create will eventually leave us with a catastrophe. Carefully planning demographics is going to be necessary for stable long term well-being. Doing that in a way that isn't dystopian is a good problem to point our ingenuity at.

The scenarios were calculated based on hypothetical 'policies' of a society and the availability of natural resources. The scenario (from the 2004 book) we are tracking most closely is no.2, i.e 'business as usual' but with twice as much resources as was assumed in the 70s.