Yes, with an extremely customised, unnatural diet, a cat may survive a vegan diet. They rarely thrive.

They rarely thrive? If you're going to make a claim, you need to prove it with sources.

But the point is that they’re unable to do so naturally, most cats will not understand why their food isn’t what they’re naturally and instinctually supposed to eat, which causes them stress.

Again, on your claim about cats not understanding why their food is "unnatural", and particularly about it causing them stress, you need to back up your claims, because it sounds like you're just making stuff up. In fact here's a study finding virtually no statistically significant difference in palatability differences for dogs and cats in vegan pet foods vs others.

"For 1,135 cats, consideration of 15 behavioural indicators also indicated that diet made little difference to the food-oriented behaviour of the cats studied, except perhaps that cats on vegan diets lick their food less often, and cats on conventional diets leave more food. However, the feline comparison groups (particularly, raw meat n = 54) were relatively small, so inference of differences should be treated with caution.

Although palatability is important, animal welfare also depends on a range of other factors. However, the results from our study, which concur with limited existing studies in this field, do not support views that vegan pet food may have reduced palatability, and thus compromise the welfare of dogs or cats in this manner."

Cats have an instinct to chew on grass for digestive purposes, but that’s about it when it comes to eating plants.

Have you ever had cats and house plants in the same home? They are almost guaranteed to destroy virtually anything you grow, to the point that it's important for all cat caretakers to learn about which plants are safe to grow around cats. Here's a cat losing their mind over a purple sweet potato. Here's one about a cat who loves cucumber. A cat who gets downright aggressive over ciabatta. A compilation of videos of cats who feverishely nom on corn.

Here are the facts. Yes, it is true that cats have adaptations that make them adept hunters of smaller animals. It's also true that there are a some essential nutrients they can't make and must get from dietary sources - if we're counting "natural" sources only, some of those nutrients can only be found from consuming other animals - taurine being a notable example. But if you think cats should only eat what's natural, does that mean they shouldn't be fed formulated cat food either? That isn't natural, they supplement even meat-based ones with synthetic ingredients just like companies do with the vegan-friendly formulations. From an article that goes in depth on the state of the science on plant-based pet foods:

"For those who feel wary about the idea of synthetic supplementation, it is worth noting that meat-based cat foods are also synthetically supplemented with taurine and other essential nutrients. This is because the high temperatures and pressures used during processing can degrade naturally occurring taurine, and other fragile nutrients. Synthetic supplementation is a standard and safe part of pet food formulation and is necessary to meet an animal’s nutritional needs."

And just prior to that:

"Indeed, to date, three peer-reviewed scientific studies using both large-scale survey and clinical data have found that cats fed nutritionally-sound vegan diets enjoy health benefits as good as or better than those on conventional meat-based diets."

What all of this highlights is that, far from being "obligate carnivores," cats are opportunists who will take whatever they can get, and the "obligate" part only holds for situations in wilderness scenarios. The nutrients they need don't care where they come from, and synthetically supplemented formulations meet their needs just fine.

And I really want to drive the nail in the coffin on your appeal to nature fallacy. Is it natural for cats to be domesticated? Some people go so far as to never allow their cats out of the house. That doesn't seem natural, but it is safer for those cats. It also reduces those cats destructive impact on the environment. By contrast, surely it's more natural for people to feed their cats raw diets, resulting in them getting bird flu, right? Like it or not, we have changed cats and dogs forever. That is something we need to think more critically about, because even now we are constantly shaping and reshaping everything they are, and everything they are going to be.

Getting a proper vegan diet for a cat is extremely hard to do and unnecessarily stressful for cats.

I literally just did a search of "vegan cat food" on Amazon and sure enough it had results. Availability might be somewhat less than the more established brands - I'm sure thanks in no small part to people like you who make unfounded kneejerk reactions - but in places where it is available, is it really any harder to just choose one bag of bits over another?

Cat food in most cases already is discarded meat not fit for human consumption. You’re not exactly saving a lot of animals by forcing a cat on an unnatural diet.

While it is true that some of the meat in pet foods comes from waste products, that's far from the whole story. You are trying to paint a baseless narrative to make it sound like feeding your companion animals other animals is harmless, when it's really not. From a study on the environmental impacts of diets for dogs and cats:

"If all of these groups transitioned to nutritionally-sound vegan diets, the numbers of terrestrial livestock animals spared from slaughter annually were estimated to be (in billions), in the US: dogs– 1.7, cats– 0.2, humans– 7.8, and globally: dogs– 6.0, cats– 0.9, humans– 71.3. The numbers of aquatic animals killed for food annually are far higher, and the use of nutritionally-sound vegan diets would also save billions of aquatic animals, in all dietary groups."

And on environmental impacts:

"Considering environmental impacts on land and water use, emissions of GHGs, acidifying and eutrophifying gases, and the use of biocides, very substantial impact reductions were associated with the use of nutritionally-sound vegan diets, in all dietary groups. With respect to land use, for example, if implemented globally such diets would free up land larger than the areas of the following nations: dogs–Saudi Arabia or Mexico, cats–Japan or Germany, humans–Russia–the world’s largest country, combined with India. With respect to water use, such diets would save freshwater volumes greater than all renewable freshwater in the following nations: dogs–Denmark, cats–Jordan, humans–Cuba. With respect to GHGs, such diets would reduce GHGs by amounts greater than all GHG emissions from following nations: dogs–South Africa or the UK, cats–Israel or New Zealand, humans–India or the entire EU."

If you’re vegan, feed your cat meat. If you don’t want to do that, don’t get a cat.

And what about people who already have animal companions from before they went vegan? And why shouldn't people consider plantbased formulas for their animals even if they themselves are not vegan? Sorry, but contrary to your belief, the status quo is anything but innocent. And while more research needs to be done to more deeply understand health impacts for other animals, that's something that can only happen if we push for it.

Plus, as cultivated meats become more available, that will take away every excuse you have. Sorry, but we can all do better, and that's not just limited to what we ourselves choose to eat. Like it or not, this conversation will not go away, and it will only get louder as evidence continues to get stronger - just as it has for human diets.

And to turn this back on topic, that goes for other animals in captivity as well. Notice that in my original comment I never made any declarations that we should just start feeding plants to predators. That would be irresponsible, obviously. I posed a question, and did so because, just like with dogs and cats, it's probably the case that a lot of claims about predator nutrition are more assumptions, and less science.

Let that sink in. The thing you are irrationally reacting to is a call for greater scientific understanding. Your entire comment is basically an elaborate appeal to tradition fallacy.