The position with the vegan cats is basically indefensible.

What do all organisms, including animals, need to properly maintain their metabolism?
Nutrients.
What are nutrients?
A bunch of different chemicals.

Depending on the specific organism, another set of nutrients is required, also varying in amount of course.

All required nutrients for humans at least can be obtained or synthesized from non-animal compounds.

From that simplified perspective, it's absolutely rational to explore how we could feed animals like cats on a purely vegan diet.
But it's certainly nothing which should be left to do for the layman alone, as veterinarian care is advisable if harming the animal should be avoided.