Having a safety critical system simply stop working is very bad design and should be punished by the relevant regulator. The design of a system has to take in t

Having a safety critical system simply stop working is very bad design and should be punished by the relevant regulator. The design of a system has to take in to account the fact that the components are going to time out and do something reasonable for maintenance. In the case of something like a fire sensor there has to be a "replacement required" interval at the end of life where the system still works and the sensors can be swapped out one at a time.