The will to live is strong. If someone denies them the food and water and they have a will to live then there is no justification.
I agree and for someone with dementia eating and drinking are often their last conscious acts of free will. The whole process of dementia is extremely difficult for the patient and all involved but, IMO, however degraded their mental faculties are it is their decision. A caregiver can only offer them the option but it is not for the caregiver to deny the option.