Taking these points in reverse order --

> Exactly none of those problems are solved by salad factories.

Honestly, I'm not particularly invested, financially or emotionally, in salad factories, but if something reduces the footprint of arable land needed to do something that was going to be done anyway -- assuming it doesn't have some large externality, like using untenable quantities of energy or producing some toxic waste product -- isn't that a net good?

> Not destroying biodiversity is. Not destroying ecological services is.

I'm worried that we're approaching this from different ethical perspectives / arguing about different things here. I think I may be working from the perspective that avoiding famines and mass starvation is the primary goal of agricultural planning, and you're worried about the ecological health of the planet as a whole.

The next couple of centuries are probably going to be ... grim for the planet as a whole, as far as the interplay between agriculture and the environment go, until we stop burning fossil fuels and the human population of the earth peaks and at least plateaus, if not declines. The best case scenario I can see on the table is that we are able to produce enough food -- with improvements in both agricultural technology, the composition of food produced, and in the distribution and storage of food -- using the same agricultural footprint, maybe being able to pull back from some of the more agriculturally marginal land and allow it to re-wild (the way New England has gone from being 75% cleared for pasturage to being mostly re-forested). Maybe a few small wildlife corridors, but no substantial decrease in the fraction of prime farmland being farmed.

The more likely and worse case is that climate change makes currently prime agricultural land ... not, and the "replacement" land to the north is substantially inferior to existing farmland (even if it has the same "growing season" of frost-free days because of warming, it will still receive less sunlight than the land in lower latitudes), so we end up using a lot more land to grow hopefully-enough food (worst case is we use a lot more land and still fall short, of course).

Most of the things that could really help here are really outside the scope of agricultural planning -- cutting CO2 emissions completely over the next decade would be great, reducing the human population would certainly ease ecological pressures, creating a perfectly efficient, fair and ethical global system of food distribution would reduce total production needs, but good luck with any of that.

> Growing food is not an issue. Not wasting food is.

So one issue to be careful with here is that the goal isn't just to grow enough food; it's to grow enough food that even in the worst-case scenario, with half the fields flooded, half in drought, and half blighted, the meager production is still sufficient to meet all needs.

Yes, we can move food from a region that is having a bountiful harvest to a region in drought, yes, we can save food from year to year. But we can also have globally bad years, where the decline in production isn't regional, and we can also have years of bad production, where droughts or floods or blights lower production for three-five-ten years at a time; centuries of cyclically good or bad production aren't unheard of in the historical record.

The only way to achieve an acceptable level of food security is to damn efficiency and massively overproduce in the average year.

A healthy chunk of the inefficiencies and waste in food production are leaning into that overproduction. Livestock can consume the extra grain calories in a good year, producing tasty, high-quality calories in return, and in a bad year, we can cull the herds of cattle and pigs, both realizing an immediate gain in calories from the livestock, and freeing up a portion of the food-stream that had previously been fed to the livestock to be redirected back to direct human consumption. Likewise with ethanol -- in a good year, you turn the extra corn into biofuel, in a bad year, the price of gas goes up and you redirect the corn to either livestock or directly to people.

You could eliminate these sorts of things, and cut back on crop production overall, and realize great gains in efficiency -- you could re-wild substantial amounts of prime cropland, even. But you'd be building a food production system that was much less secure, much more fragile -- in a bad year, there would be nothing left to cut, and it would be much easier to fall short enough for widespread famines.