Is it only an issue of nutrients? Hypothetically, what if they were able to get all the nutrients they need without eating animals, would it be alright then?
Outside of the nutrients argument which you're hyper focused on (and which I don't agree that you have compeling evidence, especially when it comes to cats), I firmly believe that the choice to go vegetarian/vegan is a personal one. Pets and small children cannot make that decision for themselves, so I firmly disagree with forcing it on them (outside of extreme cases like animal protein allergies, unavailability of meat...etc).
We definitely shouldn't breed animals for the purposes of being kept in our households. As for animals already existing in shelters it is probably the most ethical option to adopt them, but if we need to slaughter other animals to sustain them then even that is no longer ethical.
How does that matter? If they can get all the nutrients from plants why not give them food that is vegan? Their interest in taste is not a justification to violate another animal's right not to be enslaved.
The animals interest in taste certainly has to be taken into account for their well-being. And especially for cats, chewing on a tough meat-like substance is also necessary to maintain dental health.
But I don’t see how the pet benefits from the murder of another animal. There is no fundamental reason that prohibits a diet for pets that does not result in animal suffering. As long as nutritional components, taste, and texture fulfill the requirements, there is no reason to object to a vegan pet diet. But of course, not every vegan diet fulfills these conditions, and especially for cats, we are still learning how to best implement that kind of diet.
Earlier this month, a study published in PLOS ONE looked to find out if a vegan diet could be healthy for cats. Of the 1369 cat owners surveyed, 65 per cent were themselves vegan, vegetarian, pescatarian, or reducing their meat consumption. They found that there were no detrimental health effects in the cats fed a vegan diet, and even found that “cats fed vegan diets tended to be healthier than cats fed meat-based diets”.
I don't know any vegan that thinks this. This is just a way to pitch public opinion against vegans and to painting them as crazies, which they are not. Forcing carnivorous animals on a vegan diet is very obviously animal abuse.