Carbohydrate, cellulose and sugar are not necessary nutrients for humans. Fatty acids and amino acids are.
No human society has made it more than one generation as vegan, many societies have sustained on over 90% animal products.
Every society that relies on grain and non-meat agriculture suffers from reduced overall size and vigor, and reduced bone, dental and mental health, and undergoes famines and widespread disease:
In Eldoret, wealth is measured in the number of cows you have. Still waiting for the magical vegan Kenyan running superstar. Unfortunately for brain-reduced vegans, Africans are not suckers for this anti-human fad.
Just the effects of veggie/vegan diets on the brain alone (such as 1% reduction in mass/year in one study) are startling, and accounts for much of the damaged and skewed views of Sage and other veggies on this thread.
Maybe the good news is, that by the time the damaging effects of these diets are apparent to the rest of the world, your shrunken, omega 3-deprived, amyloid-plaque-riddled brain just won't care anymore. We are seeing the results in reduction of healthy fats in the western diet in the form of an explosion in dementia rates since the late '90s. Meanwhile vegans are worried about cow farts and want to tax one of the healthiest components of the human diet (especially for runners): red meat.
Not all vegan youtubers are jumping ship, only the intelligent ones who are not con-artists.