Complete thermistor failure isn't usually an issue, the firmwares usually shutdown in open or closed circuit conditions.
The bigger issue is when a thermister comes loose from the hot end or worse creeps out.
In the first the thermister continues to read slightly above room temperature and the heater stays on, you could probably detect this relatively trivially in software
In the second case the thermistor looks like it's reading constant temperature, but as it's contact with the hotend gradually reduces, the delta between it and the actual hotend temperature increases. Detecting this in software with any real reliability would be IMO hard and prone to false positives.
I've had exactly 1 thermal runaway, on my first build I did not secure the thermistor sufficiently well, over the course of the first week it creeped out of the HT silicon I used to secure it and eventually the PEEK part of my hotend melted.
It's really not a common issue, and IME it's almost always the result of poor assembly rather than component failure.