I'm sure someone ought to point out that, at least from an ecological standpoint,
Soylent Green is actually a terrific idea...
...while in real-life, err, maybe not. Although I'm sure that PETA would love that one... I'm surprised they ain't campaigning for it already.
Carbohydrates (the macronutrient most plants provide) surely have a place in the food chain (carb reloading sounds like an interesting resource), but I don't buy it that they're that good... Suffice to say that while humans can live ok without eating plants (though that'll result in the need for the consumption of other animals' organs), they don't do good without essential amino acids or essential fatty acids.
I'm not sure you can get those on a straight-up vegetarian diet without supplementation or food choices that wouldn't be available without modern supply chains, as in, food was hauled across the country, which also happens to not be very healthy.