Looks like I poked a sleeping dragon in a passing swipe about the scalability of organic agriculture in an article yesterday.
I'm not sure I'm the right person to lead this conversation, but I'd frame it as "how many people can the earth support indefinitely?"
To make this tractable I begin by making several classes of assumption about diet. So what happens with an American (meat and processed foods) diet adopted universally? A Japanese (wild-caught fish) diet? An Indian (mostly legumes and rice) diet? An organic green-yuppie diet?
Orthogonally, we should consider two cases: energy limited vs. energy abundant.
Then I'd consider the global geochemistry of agriculture. We would need to trace the mass budgets of various elements, as well as fresh water. There may be other terms e.g. representing soil health and erosion.
My guess is that without energy abundance the world is already severely overpopulated. It's not an especially well-informed guess.
I presume there are whole-systems studies of this sort and would welcome any references. I don 't feel especially competent to take this on, but then again I don't have a horse in the race.
Which side of ten billion that number lies is a pretty crucial determinant of our prospects.
I've misplaced my copy of Joel Cohen's book "How Many People Can The Earth Support" which is an admirable place to start.