Whilst dogs can exist and remain healthy on a plant based diet (though I would not say “happily”) cats most certainly cannot. Here’s a couple of reasons why (which is not my own work):
“Cats need a considerable amount of vitamin A, which they cannot biosynthesize from carotene, as dogs and humans do. Insufficient amounts may cause loss of hearing, as well as problems with skin, bones, and intestinal and reproductive systems. Cats also need taurine. A feline lacking taurine can lose eyesight and could develop cardiomyopathy.”
That is a veterinary opinion shared by most vets. Some vegans adopt their diet because of their taste but most do so for ethical reasons. Forcing a cat to adopt ethical principles followed by humans is simply inane. Cats (and other animals) do not endure ethical considerations when choosing their diet. Forcing an animal to adopt a vegan diet because of your ethics is about as sensible as a Jew insisting his pets only eat Kosher food. There is a simple answer for vegans who object to their pet cat eating meat – they should not keep a cat.