The question is how eating an unsustainable amount of vegetative matter came to be considered healthy to begin with.
I was paying attention when the first modern phase began, via the religious diet of a President who died of that diet, and a biased science popularizer with a degree in philosophy. This was used, along with the "carb-up" notions of 1960s and 70s narcissistic exercisers, to push a high fruit, grain, and vegetable diet, riding on the correct advice that SOME greens are good in the diet of many people.
I also happened to be involved in the second modern phase, as I worked in the healthfood industry during the late 70s and 80s. That was also based on a religious push, but this time Hinduism. A bunch of American Yoga enthusiasts took the worst parts of a small portion of India's culture, and made it a false god. Ignored was that most Indian, so called vegetarians, ate plenty of meat - goat, fish, birds, insects etc.
Man! We sold a lot of vitamins to people! After telling them to eat low nutrition foods only (of course, we thought they were high nutrition, but we were wrong), we came back around and told them that now they needed laboratory chemicals to make up for the lack of meat.
This current phase (2005-present) is largely caused by corporatism as a defensive attack against Low Carb and Keto. The fact is that canned vegetables and dried grains are cheaper to buy, ship and store, and they sell at a high markup to "plant based dieters" as "a healthy part of a balanced diet!". Add to this the false idea that proper pasturing of food animals is bad for the environment (Global warming hoax) and you have a fake "counter-culture" of macaroni and tofu kids ready to become the grist for the industrial agriculture and pharmaceutical mill.
The truth is that having SOME greens is good for SOME people. And for those who were living on basically beef and potatoes in the 1930s and 40s, having a salad now and then might have been a good idea. But eating excessive amounts of roots, fruits, and grains that need to be fertilized with fossil fuels and shipped around the world to be available to people who never would have them in a real natural existence, is basically wrong. That IS bad for you.
In fact, most of the extreme anti-plant-food folks are probably those who (through wrong diets growing up, or because of epigenetic shifts caused by their parent's vegetable based diets), have damaged metabolisms which cannot handle even the small amounts of greens most people can, and maybe SHOULD have. Of course they are most vocal, most controversial, and because they are (somewhat) outliers, they can be the easiest to refute.
That doesn't change the fact that the most successful cultures have always been those that ate an animal based diet, with a few greens.