At a certain point the fatness doesn't matter.

I can basically guarantee you that this cat has untreated diabetes and will die from it, if CKD doesn't kill it first. But that is true of much smaller fat cats. By the time the animal has ballooned up to big proportions, it means years of eating shit dry food and almost certainly not getting enough water. So if they don't have diabetes yet, they probably have kidney issues already. I guess you could give your cat insulin everyday... but I doubt many people are about that...

Cats aren't meant to eat dry food, they barely drink water in nature and get the vast majority of their liquids from food.