Plus evolutionary history shows plenty of examples of animals switching from pure carnivore to pure herbivore to omnivores in between. All birds are descended from a common carnivorous ancestor, but plenty of birds today subsist mostly on seeds or fruit.

If there is a lot of available biomass to be eaten, nature will find a way and some animal is going to fill that niche. Many of the folivores (herbivores specializing in digesting leaves) that descended from carnivores have to deal with the low nutrient/calorie density of their foods by just eating a lot of it, and have varying levels of microbial symbiosis for helping with that digestion.