I agree with your comment entirely concerning the healthiest populations and the fact that one can’t forego meat completely. I would add that the changes to diet for just about every culture have changed radically, not just a little bit but a lot. Historically, people never ate processed foods at the level that people now do. Also, for many cultures (not all), the invention of refrigeration has meant eating meat at a rate that is far higher than their ancestors ever imagined. Ditto for the advent of the entire transportation industry and the changes it has meant to diet. Lastly, most folks just simply don’t get near the exercise or burn off calories at the rate of years ago. What we are looking at is a grand experiment on all of humanity....
Human population exploded with farming and the availability of grains, but prior to that isotope evidence suggests that human diet was, at times, virtually carnivorous. The last 5000 years is the anomaly in the greater half-million-year history of our species. And while it has been good for human population as a whole, it hasn’t necessarily been good to us on an individual level.
Our ability to extract more fat and nutrition from prey than other carnivores supercharged the growth of our brains, and contrary to being the smartest humans in world history, our brains are 5-10% smaller than our pre-agarian ancestors.
As to the calories argument, a fat heavy diet is more satisfying than a carbohydrate and grains-based diet. As a result, people on a meat heavy diet that severely restrict or eliminate carbs from their diet can rapidly lose weight. I speak from personal experience on this.
As a middle aged man with a wife and kids I am certainly no more physically active than I was 20 years ago when I was young and single, but my BMI is down to 23 from 28.6 and I feel much more energetic and healthy than then. The major change I made was to cut out sugar, fruits and starchy plant based foods and move to a diet high in natural fats (butter, lard, tallow, and EVOO). I am more rested with less sleep and no longer have acid reflux.