Earlier quoted context omitted.
It would evidently be bad for people to be blobs of fat, remember that in WALL-E they couldn't even stand up. That level of physical impairment is awful whether you're mortally ill or not.
Why? If I value eating whatever I like above having a small waist, why would it be bad for me to be a blob of fat? The only reason I should lose weight is for health reasons, but if there's a magic pill that would remove that reason, then why should I care? I kind of agree with you, but I don't think it's immediately obvious.
We already see those 2 problems right now: Existing issues about how to handle people who don't fit in economy seat on a plane without disturbing the next person. Smartphone (and previously calculator) design doesn't work for people with bloby fingers.
We barely started to make essential services available to disabled people who don't actually have a choice of moving on their own, or read, or... If we can't handle that special case, I don't think we can easily handle those self-inflicted issues as a norm.