I don't know what man was born to eat. Nor do I know that this sort of argumentation is actually terribly conclusive or useful since we have very limited knowledge of what man was born to eat or what man's body was designed to handle best. As far as carnivores go, since people eat meat, they are at least functional carnivores and they don't have the sort of digestive system that you describe. Nor do I think that dogs do, either. They may gnaw on bones to get at marrow, but much prefer soft tissue. Large cats and some reptiles are equipped to tackle some digestion of bone. Canines and hominids don't seem to be able to do so, and yet seem to do fairly well avoiding bone. We have an adaptable digestive system as we have a fairly adaptable brain.
I'd like to see some societies that get along well on diets with under five percent protein, myself. I don't know that we need protein levels as high as those that we actually get in western society, but I don't think that levels as low as five percent would be healthy for a very long time. If you have nutritional research rather than simple assertions of truth to base this on, I'd be pleased to see them. I know that when I was practicing aikido all those years ago, we had a number of vegan and macrobiotic folks who studied at the dojo. My observation was that while they seemed to have at least as much energy as I did and perhaps more, they also tended to bruise easily and took a long time to heal from both bruises and other injuries, such as sprains and tears, and that these injuries happened to them at a somewhat higher rate than they did to the non-vegans and non-macrobiotic folks.
I was envious of their energy levels and of their ability to keep a trim weight. I was not envious of their record of injuries and of their somewhat longer than usual recovery time. I don't know what your observations have told you about this sort of stuff. The macrobiotic diet has always been reported to be useful in dealing with cancer. I have never seen that tested, and I'm not sure that any real test would be ethical. Settling that question would probably have to be done on the basis of retrospective matching samples or some such. If you could find identical twins with the correct variables, that would be useful.
I think that actually living a vegetarian lifestyle is something that has only recently become a possibility, when fruits and vegetables have become available out of season to people who want to consume them. Otherwise, one ate what was available from what one could either grow or buy, and the means of storage didn't allow many fruits or vegetables to survive the winters in good shape. Perhaps root vegetables in a root cellar, or carefully packed apples or some canned fruits and vegetables put up at home. There was good reason to depend on smoking and salting for meat and fish. They were an important part of a person's diet, as was bread. Fortunately legumes kept well in dried form and when mixed provided a source of complete proteins. There were fairly constant problems with vitamin deficiencies, however, because of the lack of fresh fruit.
Not many people could actually afford to give up meat or fish.
My wife and I haven't been able to get back to England in almost five years, but the last time we were there, when you saw a salad bar in most restaurants, they were almost universally white, filled with Potato salad, and macaroni salad, and perhaps tuna salad and egg salad, and they were almost universally bereft of anything that looked, smelled, or tasted like a vegetable. I hear there is change afoot.
On our honeymoon, we spent a week on Skye, where we ate at a vegetarian restaurant with a recommendation by the vegetarian society. I ordered Pasta Primavera. It turned out to be mushy spaghetti with overcooked peas in a white sauce, all spooned over rice. The side vegetable was an order of roasted potatoes.
Evidently, somebody had read them the vegetarian principles from a book of theory, which they roundly agreed with, and sought them to adapt them to the best of English Cuisine. The spirit was enthusiastic. They needed a cookbook.
I tried vegetarian life for a few years and enjoyed getting away from meat, since I tend to like animals as a whole, but I found myself relapsing. I try to keep meat itself at a reduced level in my diet, though I do eat it, and will eat eggs and milk products. I do my part in poultry birth control in this fashion, and cheese and milk punish me for each and every transgression, so we have reached a wary compromise.
I'm wary of saying that everybody who wants to eat meat should have to go out and hunt it themselves, since I get nervous about people with guns stalking about the terrain. I'm never sure where they'll draw the line as to what or who's in season at any particular time. I'd rather have people do it that don't have so much fun with the process. I'd rather keep joy and killing with a discrete social gap between them as much as possible. You never know.