The extent to which people eat in a way poorly aligned to their genetic make up, and that if fixed they feel much, much better, is hard to overstate. If you were genetically tooled to eat nothing but fatty fish in the tundra, this may work great. And eating bread if you aren't from the breadbasket may not be. Even vegetables tend to be regional, and many can be pro inflammatory in certain people (ie. Nightshades), which leads to malabsorption and therefore a lack of nutritional value in practice. Chances are eating to your genetics beats eating any one particular way. People just like the idea of one diet to rule them all. The trade off with meat seems to mostly involve shorter lifespan due to eating more calories and burning through telomeres. If someone offered much better mental health for your entire life and it takes five years off your life expectancy, you might do it, and that's mostly what's at stake here.