It's a shame; i'm sure there are vegans feeding their cats this way, and when those animals lose muscle mass quickly, the first thing that gets really damaged by that are their kidneys - and this does normally only get noticed shortly before the cat is going to die.
And it's an ugly death.
I've had a young cat which had nearly dead kidneys when we got her, and it's pure torture for them - we tried everything we could, but there's not much to be done after they show symptoms.
Those "studies" you are throwing around with the owner-reported feedback regarding the health of their cats which can only be objectively be seen by bloodwork and a kidney ultrasound have actual negative worth.
yup, opiates to be specific.