We're already growing more calories per acre than any time in history, by a large margin. Mostly doing the very agricultural practices you decry, the ones that are "destroying the planet". That's one reason the population has grown so rapidly - famine is no longer keeping it in check.

"The last shreds of wilderness" is an absurd hyperbole. There's still a lot of wilderness left, and I think we're approaching maximum land under cultivation. (checks google) As a matter of fact... in 1970, 4.48B hectares were cultivated. in 2008, it was 4.83B, a <10% increase over almost 40 years and a near-doubling of the population. What happened? Incredible increases in yield starting post-WWII, greatly increasing the amount of food per acre.

So where does this leave the original study? Does it assume there will be no improvement in yield? Is farming more land the only solution being tried? Obviously not. What they're breathlessly recommending is exactly what we've been doing for over half a century now (really, about two centuries) - improving yield.