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).
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”.
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.
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.