Terrence George, 72, died in January 2016 at the Royal Cornwall Hospital after an attack of pancreatitis caused by gall stones.
He was taken to hospital with mild pancreatitis five months before, but had not had his gall bladder removed.
Doctors acknowledged it could have been successfully treated at the time by the procedure, an inquest heard.
Senior Coroner for Cornwall, Dr Emma Carlyon, was told Mr George had been discharged from hospital a week after being admitted, pending a six-week review, but there were delays.
The inquest heard that because of a "catalogue of errors" and "systemic failures", he never had the potentially life-saving treatment.
At the conclusion of the two day inquest, Mr George's family said they were "devastated" by his "entirely unnecessary death".
Whilst concluding the case, Dr Carlyon said she would be writing to the National Institute for Health and Care Excellence, urging them to introduce new guidelines in treating pancreatitis.
New guidelines for care should be prioritised following the death of a grandfather, a coroner concluded.