Nobody is suggesting we feed cats tofu and spinach. The naturalistic argument and yelling "We don't know!" a bunch really only works if you are proposing we feed cats only raw meat from fresh kills, what they would eat in the wild. Pet food isn't a pet's natural diet, vegan or not, and it all has to be fortified.
I don't believe in such a naturalistic argument though. Humans are able (key word, able, most don't) eat healthier with modern diets. Why would we assume we can't develop food that is just as healthy or even healthier than an animal's natural diet?