I have a read only temperature sensor in my old-school non-computerized refrigerator and freezer. Hack until the cows come home but it still can't turn off my freezer.

Air-gapping, network segregation, data diodes, etc. are all real things. It's possible to make a read-only refrigerator that is "unhackable". I have a temperature monitor in my refrigerator right now. Hack that monitor and it doesn't affect my refrigerator at all.

not in this case where a thermosensor can do nothing but monitor a temperature.

Yeah? We had an old Unix server that printed its logs in realtime to a dot matrix printer. How are you going to gain control of that?

Read-only is a strong control with lots of solid implementations.