Other presumptions, including irrebuttable presumptions of law, such as the age of criminal liability, belong more properly to the substantive law.
This creates an irrebuttable legal fiction unconnected with the facts.
There is without doubt a very strong presumption in favour of a course of action which will prolong life, but… it is not irrebuttable.
There would be a fixed list of relationships covered by the statutory duty of care, creating an irrebuttable presumption that there was a close tie of love and affection.