As a vegan, I am against breeding animals to be pets; companion “domestic” animals should only be rescued ones, and the aim should be to phase out the pet industry and mentality altogether. But the reality is that astronomical numbers of purpose-bred animals are living, and being bred today. The obligate carnivores among them are being fed a small fraction of the body parts of a far more enormous number of animals that are being purpose-bred for human consumption.
By not eating meat, vegans (still well under 2% of the world’s human population today) are, through a “trickle-down” effect, reducing by a tiny amount the total number of animals being bred and slaughtered for consumption. But it is by working to transform the other 98% of the human population by informing them about the enormity of animal suffering and the safety and health of a plant-based diet for humans that vegans are helping animals, especially in the future — not by trying to make their cats vegan.
Both sides of the question agree, as they must, that the long-term health effects of a vegan diet for cats are not certain. Short-term health is no proof that it will not sicken them eventually. And even if it were possible to keep them healthy, it would clearly need life-long blood and urinary testing of the sort that does not scale to all vegan rescuers.
Vegans need to look into their hearts to ask themselves whether they are doing the right thing for animals whose lives and health depend on them in taking risks with their health instead of giving them the benefit of the doubt and working instead to open people’s eyes and hearts to plant-based diets for themselves rather than their obligate-carnivore rescues.
Yes, being vegan is not just about what people eat, but, again, working against animal breeding and selling, and for the adoption of rescued animals only, is far more likely to help far more animals in the long run than taking chances with the health of purpose-bred cats who did not ask to be human pets, and who depend on our judgment and care for their survival.
The same applies to the humane care of large obligate-carnivore rescues (lions, tigers, seals, orcas, alligators) in sanctuaries today. Sterilization is far more merciful to all than putting their health at risk. (But none of this rules out the importance of continuing research efforts to synthesize animal protein without harming animals.)
But see also: https://www.facebook.com/harnad1/posts/10211472145830032