You wrote, "Detecting a malfunction in a sensor is hard, really hard."
Actually is is quite simple to do. If you get anything more then the cheapest of sensors, they continually diagnose themselves and report back the diagnosis. There are failures that cause the sensor to freeze up and stop reporting. If it keeps sending the same data, easy to detect the value stopped changing. If it stops sending any data at all, easy to see a step change that should not have occurred and you also do a redundant sensors and do 2 out of 3 voting. I could continue with scenarios, but while it takes some work, some program and some costs, detecting a malfunctioning sensors is easy. Cars currently report malfunctioning sensors, most of them with a check engine light.
Not sure of your background, but I am a Controls Engineer. I do this for a living. I am sitting at a plant where I have over 20,000 sensors of various kinds and detecting the fact that a sensor failed is quite easy to automatically detect and alert the operator of failure.