I take your point about our micronutrient requirements, but I don’t think it’s all that difficult to meet them (unless a diet is really weird). You’re right a little variety is useful, and that’s why I like the traditional “four food groups” philosophy. Whenever I put together a meal with meat, dairy, a veggie, and a starch, it ends up looking excellent on a nutrition calculator.
Of course, there’s nothing wrong with eating a variety of meats, or a variety of veggies, or whatever. I just don’t think it’s necessary if you prefer simplicity.
In fact, my usual lunch, which is basically just potato chips and cottage cheese, provides almost everything by itself. A hundred years ago, the Irish lived on dairy and potatoes, and the diet was fine nutritionally until the potato blight (and then the lack of diversity became a problem). That kind of narrow diet wasn’t so uncommon in traditional societies. Even today, I’ve know plenty of people who grew up healthy on chicken nuggets and hamburgers. There are a few crazy people who live on nothing but meat, and there are people who have fasted for long periods with just bread and water. Eventually that bread-and-water group will have problems, but (in my opinion) it’s pretty difficult to develop a micronutrient deficiency eating a modern diet, especially with white flour products being fortified.
I think the main concern with a junk food diet is a low protein intake relative to calories. But if you eat junk food moderately for half the day, and have a big cheeseburger for dinner, then I doubt you’d develop any deficiencies. Meat and dairy are both loaded with protein and minerals.
I agree with you that much of Ray Peat’s dietary philosophy is silly, but not because of deficiencies. It’s just unnecessarily restrictive and encourages obsessiveness. And you’re absolutely right that people will eat greens when they’re available. Their mineral content is especially useful for cultures (like the Chinese) that don’t have as much access to meat and dairy.