Florian Habermacher's Quick takes

Research vegan cat food as ideal EA cause!? Might also be ideal for human vegan future as 'side'-effect too.

1. Cats are obligate carnivores; must eat meat (or animal products) according to typical recommendations (and cats tend to refuse most non-animal foods). At least, there seems to exist no vegan cat food that is recommended as a main diet for cats without further warnings; often cats would seem to not accept mostly non-animal foods
2. I guess - but am not sure (?) - animals fed to cats mean significantly more animals are grown in factory farms
   1. Somewhat counterintuitively, in the whole cat food domain, the concept of animal welfare standards, does not even seem to exist. You can find some seemingly higher-welfare standard products but they are extremely rare
   2. Even if often large shares of the ingredients are "chicken meal", "fish meal" etc., I guess lots of this meal one way or another still could have been replacing some human foods in some places. What I have definitely seen, often major shares of ingredients in the cat foods are "meat" and not just inner organs or broth (although I cannot exclude that 'meal'-based ones may dominate total sales volumes)
   3. I guess we're pretty good feed all sorts of animal pieces to (i) ourselves in sausages, chicken nuggets, and the like and/or (ii) other food industry animals. So my prior is that cats do not only get stuff that is completely redundant in the food industry.
3. I calculate* (very rough) for ca. 220 mio. house cats worldwide, and considering 50% of their meat food to correspond to extra meat production, 6 600 ton/day quality-adjusted meat consumption, or around 0.9% of human's meat consumption.
4. The few articles I read online about to which degree cats require a meat diet, point mainly to elements that sound like those that we can easily mix/synthesise from non-animal foods and chemical processes (Taurine, Vitamins A, Arginine, Niacin, maybe some other fatty/amino-acids)
   1. Oddly, the pages tend to list these few elements, insisting that therefore the cat must eat meat, while I'd think: "Euhm, if it's just that, it would seem simple to mix the right thing" => maybe the pages just do not enter into more subtle details that are crucial for an obligate carnivore
5. IMHO, we could very easily test out food/supplements mixtures to check how easily one can replace which share of meat for cats without impairing their health. Given the billions of factory animal lives at stake, even some risk for the corresponding "test animals" might be completely justifiable in the worst case, and naive me thinks we might make extremely quick progress on this front if we really want
6. If we nail this, the positive side-effects could be: "Hey look, they even feed the obligate carnivores with this mix nowadays, surely you can also become vegan with zero hesitation with a human adjusted formula!" - i.e. finally the stories of your vegan friend who end up at the doctor who recommends him to eat meat (!) etc. could finally really be stories only of the past. (I know many think it already is; maybe you're right; but I know in practice at least for many this is simply not how they see it)
7. In fact, for each of (i) the cat-not-eating animals, and (ii) side-effect for human diet, I'd not be surprised if expediently trying to get vegan food that even cats can eat, would be justified

EA dietitians, am I just naive or could this be a thing?

I reckon one drawback of an ideal vegan cat diet could be that many more might want to keep cats. I see some possibilities on net impact from cats+food directly then:

1. Only few more cats: lower net animal consumption and lower net land-use and lower food costs for poor people (and for cat holders)
2. Much more cats: vegan diet more than offsetting the spared animal food industry footprint, i.e. larger net land-use change for agriculture, higher food prices for poor

Whether house cats are at all net "happy" or not, I do not know.

* Calculation, based on rough values:
220 mio domestic cats (ignoring 480 mio stray)
3 kg avg. weight (might be slightly low side)
2% of cat weight meat food/day
=60g/cat daily meat = 30g/cat daily "extra" animal meat if quality-adjusting with 50% (see text above)
=6 600 t/day extra meat production

And with approx. 90g meat/day per human (beef veal pork poultry and sheep acc. to OECD) for the 8 bn humans, i.e. with 750 000 t meat/day human consumption, the cat's share is

= 0.9%, bit simplistically approximated.