Sandy:
One of many handy features of having pilots on board aircraft is that when an engine overspeeds, the pilots retard the power levers, or shut down the engine, depending on what mode of overspeed in being experienced. The various flight manuals, systems composition, normal and emergency procedures, and application of checklists are practiced and learned for a very good reason: an engine's clever little electronic and hydroelectronic and mechanical bits may go wrong.
Most aircraft engines have an overspeed governor of some sort, some have more than one. Some are mechanical, some are electronic.
I will suggest to you that allowing the robot (which can go wrong) to kill your engine without your input is a good way to lose an engine right when you can't afford to.
Respectfully, I find your appeal to "perfect automation" an appeal to unsafe flying.
@ no-hoper: thanks for your list. I may have overstated the case, particularly in re Power Turbine overspeed.