You make a good point, but in the end the responsibility is on the life-critical application (e.g. medical software, device, self-driving car) to ensure that is has been properly QA'd and that all of its dependencies (including any cloud services or framework that it is built upon) meet its safety requirements. The event of an app server or cloud service experiencing downtime would very much have to be planned for as part of a Risk Management exercise. Ignoring that possibility would be negligent.