Firstly, my general approach to this problem is to worry about it later, because obviously it's not a problem at all in a world of 8 billion humans all wanting to eat meat every day.
But, since you seem to know what you're talking about, what do you think would be the minimal amount of animals and land required to feed those 8 billion organically? Assumptions:
- animal manure is absolutely required in the absence of synthetic fertilizer (if true, I did not know this, I assumed that a forest could renew itself without the help of fauna)
- all 8 billion are willing in theory to go vegan
A rough picture of what that would look like? Lots of cereals and legumes and so on, plus a couple of chickens per hectare?
Interesting. That certainly looks like a better world than the current one.
This model seems to be optimizing for a specific conception of human nutrition and wellbeing. Fair enough, that will definitely be an easier sell than veganism (if still extremely hard due to entrenched interests).
Personally (like many others here) I would prefer to go further still and optimize the model for biodiversity and animal wellbeing. 40% of current US meat consumption is still pretty high, seems it would be possible to cut that much more without conceding any ground on human nutrition. All of our nearest ape cousins are heavily (if not absolutely) vegan. That to me offers a pretty big clue about what's possible and even advisable.
In this alternative model, I suspect the bottom line for the animal biomass necessary for manure would be above the bottom line for optimal human nutrition, and lower than the figure necessary to produce a kilo of meat per person per week. Especially if it involves lots of egg-laying manure-producing chickens instead of large grazing ruminants. Such a model would require less land still. And if there's one thing even better for the environment than a best-practices agroecological farm with well-paid cooperative workers, it's no farm at all and a forest in its place.