What about a low-junk food diet? No cqlorie restrictions but no refined carbs? I don't think we need ti discourage people from eating whole grains and beans.
As a prepper and a homesteader (amateur/beginner), I realize that low carb and high fat/meat isn't really feasible when SHTF, and I mostly stock lentils, pintos, rice, wheat, quinoa, and a few other standard staples. They stay good, when properly sealed for like 25 years (quinoa only 8 years or so though...). No meat known to man (or at least myself) does that. Also, even doing basic homesteading, you tend to eat more veggies and much less meat.
In the meantime, I admit that I eat plenty of meat, and could probably raise a cow or some pigs, or at the very least do the rabbit thing, but it's a lot of work and planning, and it's just easier to get your meat from a can in an emergency situation.
Edit: I prep for emergencies, FWIW, not health. Calories and nutrition are the major requirements.
But they don't, and the industry is massive.
Sustainability only makes sense relative to a particular population size, and so something is only unsustainable in the absolute sense if the population needed to produce it exceeds the population which can survive upon it. This is not the case with the food types you refer to, and the producer/consumer ratio is precisely the thing that we develop new technologies to reduce.
If anything, I see it is better for us to engage in inefficient land use, because it provides spare capacity which can be used to mitigate prolonged disasters; a view strongly influenced by the book "The Collapse of Complex Societies", by Joseph Tainter.
start eating animal organs - liver, kidney, tongue, brain, throat, face meat, bone marrow - see how much food is actually in an animal.
Paleo is pretty affordable at the lower-middle class, if you're willing to put the effort in and cook. My biggest hurdle is how easy unhealthy stuff is.