would seem the most logical and safe.
Logical perhaps, but not neccesarily safe...
The Spanair accident in Madrid was (partly) caused by such an automatic failing.
It probably is a philosophical question, but having the pilot in the loop will increase awareness, any automation creates a potential for being taken for granted, and being missed in case of failure.
So you have to add an automatic monitoring and warning to the automatic pitot heat, just increasing complexity.
it's not rocket science to have a sensor in it for temp reg.
It obviously is... There were such devices many years ago, but their reliability was poor, so everybody switched back to the dumb heatings... Maybe today with solid state devices being able to handle the high power a more reliable, integrated temperature control would be possible.