I can see through your logic and chose not to engage in a verbal tennis game based on a illogical fantasy. Animal eats plants and they thrive. That is how they live naturally. Surely we can survive off of what they eat too. 

Human physiology adapted over millenias to consume both plants and animals for energy. We run more efficiently with both in our diets. 
Simple, true, and undeniable. Just the facts.

Can we survive off of just plants and no meats yes. Can we honestly thrive off of just plants and no meats, not really. We will always be lacking in vital energy which comes from animal fats. That only real meat can provide us on demand with maximum efficiently. 

However, there are people who will try and convince us otherwise without openly admitting to the negative side effects. They tackle with moral conflictions, because of their misunderstanding of the natural world and their place on the food chain. This is the result of being so pampered from modern amenities that they no longer have to worry about having enough to eat. 

We are hunter gatherers. It is in our nature and instinct to hunt, kill, and eat animals. As well as it is to cultivate plants to harvest to. Choose to find splice in this balance, or live a life filled with delusion and high probability of  constant malnutrition.

Not knocking peoples life choices. You wanna be a vegetarian or vegan, you do you. Be happy.  Just being brutally honest with my understanding of scientific reality.  

 What specific physiological adaptations have taken place? This alleged instinct to hunt, kill and eat animals, what does that feel like? I have never felt that. The instinct to pick fruit off of a tree and eat it, I can relate. High probability of constant malnutrition? I am 64, the last 36 of which I must just be lucky to beat the odds living my illogical fantasy. 

 Bless your heart. Good for you little buddy. Have a wonderful day.