That does not make any sense. Old people’s healthcare gets paid by the US government, literally everyone up and down the healthcare chain earns more profit by extending someone’s life who would rather die.
Meanwhile private insurers definitely deny coverage to people with chronic illnesses (not everyone who is sick or dying is on Medicare/Medicaid). In a single-payer system those folks wouldn't slip through the cracks and .... the doctors would be incentivized to keep them alive! (assuming doctors are profit-motivated sociopaths like your average HN poster)