Most times the lung transplant only lasts 5 years because the lung is already old when it comes from the donor and also the lung transplant decides not to take properly.
So then the patient may need a new lung transplant in 5 years or they may die if they do not get another lung transplants.
Having lung transplants does not mean that the person is cured of the lung disease and will continue to live a long life.
However the lung disease is basically gone with a good lung transplant but it doesn't mean that the person will survive much longer afterwards.
Some people even die within a year after the lung transplant while others may live only 5 or 6 years.
It's usually because the lung is not able to cope and wears out faster because it was not actually meant for that persons body.