//If you have something not too difficult to cite, then I'd appreciate a glimpse.//
Unfortunately I don’t.
//Humans can undoubtedly survive on vegetarian diets (as could dinosaurs and Hitler and can cows and fighting bulls)//
We’re talking about a vegan diet, not a vegetarian one. Humans are not dinosaurs, cows or fighting bulls and there is an argument that Hitler was not human. The cows and bulls have considerably different digestive systems to humans and whilst I don’t know for sure, I imagine plant eating dinosaurs did too.
Whether they can survive healthily is open to considerable debate. Here’s one article that suggests they might not, even with considerable intervention in the form of diet and supplements:
https://www.cleaneatingkitchen.com/vegan-diet-dangers-health/
But the most important reason for opposing the imposition of a plant based diet as the protesters seem to insist on is that many people wouldn’t choose to. If some people are worried about mankind’s survival, the best course of action is to persuade people to reduce the global population. Then it wouldn’t matter too much what they eat.