Yep, we have the least efficient food production system ever devised in the entire history of civilisation, by several orders of magnitude.

Personally, I see that as a cause for hope - it means that we have ample opportunity to improve it. We've gone down the brute force route simply because that was cheapest and easiest (well, that and a few historical accidents). When brute force solutions are no longer cheap or easy, then perhaps we can start trying more elegant approaches (I'm thinking Permaculture here).

As for slaves, I doubt they'll be needed. If the choice is work the land or starve, I know which one I'll be choosing. I've lived the life of an agricultural labourer before, and it really wasn't all that bad.

Yes, except as per our "let others do the work" ethos, the guys with the guns who don't want to muddy their hands, will take it from those who actually farm. By force.

This is the way it has been in the history of food production in times of scarcity and empires.

If things come to scarcity again at global scale, I think it is fair to assume that the same cause of action is at least probable.

Here's hoping it won't come to that.

Reading the One-Straw Revolution (Fukuoka), one can't help to think that a more efficient production system is possible, but it will take time to learn for most and even longer to transition to it. That is, if energy scarcity becomes so bad as to warrant a complete makeover of the "totalitarian agriculture" as Daniel Quinn has labeled it.

slaves? who need them if the robotics can do their jobs more cost effectively?
guns? who need them if the designer virus can do their jobs more cost effectively?