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

Cats have an instinct to chew on grass for digestive purposes, but that's about it when it comes to eating plants. Getting a proper vegan diet for a cat is extremely hard to do and unnecessarily stressful for cats.

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.

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