There are two main reasons why meat, dairy, and cereal products have come to dominate the dietary landscape. First, humans have a natural taste for animal-sourced foods, a taste we've gradually become more accustomed to indulging as our prowess in hunting increased, a skill which then became less relevant with the domestication of herds and flocks. But secondly, and for the purpose of explaining the current situation, more importantly, the combination of cheap oil and mechanized agriculture has made grain production so inexpensive that the use of grain in animal feed competes with its use in the production of ethanol for fuel.
The other side of that coin is that, so far, mechanization has mainly been applied to grain, soya, and a small selection of other crops that can easily be handled in bulk. Meanwhile, until quite recently, crops like strawberries, tomatoes, and asparagus had to be harvested by hand, making them relatively expensive. Mechanical handling is now becoming available for the more common fruits and vegetables. On the other hand, these crops are also typically heavily dosed with synthetic fertilizers and pesticides.
But mechanization as currently practiced comes with a significant downside. The machines, or at least the tractors that pull them, tend to be large and heavy, compacting the soil over which they pass, which must then be tilled to reloosen it, which kicks up some dust and exposes the soil surface to further erosion, as well as over-aerating the top few inches of soil, resulting in accelerated decay of organic material. The loss of organic material reduces the soil's capacity to absorb and hold water, making production more dependent upon predictable weather, something we can no longer count on.
The solution to all of these problems is to replace conventional mechanical approaches with smart machines, robots, that get their energy from sun and wind, use techniques compatible with no-till polyculture incorporating perennials, and operate autonomously. Using this approach, biological methods could supplant most use of synthetic fertilizers and pesticides, and to the extent these were needed they could be applied sparingly, with precision. This approach would also act to level the pricing of fruit and vegetables as compared with meat, dairy, and grains, making a more balanced diet more affordable.
One additional benefit of the robotic approach is that it could just as easily make room for native flora and fauna, making the same land serve both as native habitat and for crop production. Beyond a certain level of sensory and mechanical sophistication, it becomes a matter of programming to take such factors into account.