Tbf I don’t think the vegan pet food is just veggies with taurine added - it sounds like a manufactured protein based meat substitute, with the major nutrients being added. But vegan meat substitute is still not meat - you can tell by comparing the nutritional information of quorn with beef or chicken. In humans that’s largely fine* because as omnivores we have the potential for huge diversity in our diets, but that’s not so much with cats. Who is to say what other nutritional qualities of meat are really important, which haven’t been added after the main ones?
I’m also aware from the dieticians at work that food supplementation is fantastic but it’s still not a substitute for actual food, for reasons that aren’t still fully understood. I remember something about fibres (not the type that helps you poo) but that’s as far as I got.
I’m not saying that there couldn’t be a well balanced fake meat that cats can thrive on, but it would need more analysis than a short self selecting survey of pet owners.
*unless you’ve got/your genetic family has got a history of nutritional deficiencies or gut issues
I mean, I do once remember a cat tucking in happily to a vegetarian curry* but generally as a species, they’re pretty much designed to eat meat. Perfectly so in fact 
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