"However, there is little evidence of adverse effects arising in dogs and cats on vegan diets. In addition, some of the evidence on adverse health impacts is contradicted in other studies. Additionally, there is some evidence of benefits, particularly arising from guardians’ perceptions of the diets. Given the lack of large population-based studies, a cautious approach is recommended. If guardians wish to implement a vegan diet, it is recommended that commercial foods are used."

Thanks for linking the review! Of course more and better research is needed, but a cautious approach like they recommend makes sense to me. If you feed a cat a vegan diet, you need to monitor their urine for crystals (there are special litters that do this, or stuff you can put on their regular litter), and you should know you can't switch them overnight. But it is possible for cats to be healthy and happy on a vegan diet.

Your intuition that vegan cat food is as healthy for cats as ice cream sandwiches are to humans is not enough, and when you look at the nutrition content it doesn't even make sense. An ice cream sandwich with all the necessary micro- and macro- nutrients a human needs to thrive would be a better comparison, and guess what, a nutritionally complete ice cream sandwich would be fine! I've gone for long stretches where I eat nothing but meal replacement shakes like plenny and huel because I'm lazy, and it's fine. You can find at least one person on reddit who's done it very strictly for years with no problems.

if you're going to fight this battle, you also need to fight it against non-vegan cat kibbles, many of which are worse for cats (i.e. less nutritionally complete) than vegan kibbles like benevo or evolution