There are two main reasons why a replaced lung doesn't last forever. One is the immune system and (best case slow) rejection of the transplanted organ. The other is the same reason why the original lung has failed. Normally that was a disease that also attacks the new lung. https://en.wikipedia.org/wiki/Lung_transplantation  Transplanted lungs typically last three to five years before showing signs of failure.  That's a slow process but after 10 years a failure is very likely.