Very true. But if they’re morbidly obese as it is, it’s pretty likely they have a brain chemistry which responds very positively to sugary drinks, so without the diet soda it’s possible their health would be even worse.
I’ve never met an obese person who liked being obese, but I’ve met lots who have a really challenging relationship with food which slowly brought them to the point they’re at now. I’m an addict as well, it’s just not as visible.