Re: As a farmer/rancher ...
<...."GPS fails, the thing will not stop"....>
That is at least partly correct - as you say, current GPS steering systems don't actually stop the machine if the GPS fails or the signal drops out, but on all the ones that I have used, loss of signal results in the sterring turning fully in one direction, so the machine will continue to go round and round in approximate circles until the driver grabs the steering wheel.
They still require a driver to turn the machine around at the end of each run, and still mostly require driver input to initiate any auxiliary functions involved in the turn (lifting/lowering the implement in or out of work, switching auxiliary power take off shafts in or out of gear etc., but these functions are usuallu set up usinmg the tractors' own headland management system and once set up for all functions usually involves a single press of a button on reaching the end of the run, and another single button press once the tractor is aligned with the next GPS A-B line (which is programmed to the working width of the implement).
Apart from watching what the machine is doing and getting off to make adjustments (or for 'anciliary' reasons ;), once it is all set up and working as it should, the modern tractor driver doesn't have a lot to do, which (particularly when working long hours) can result in becoming inattentive or asleep, and then this sort of thing happens:
https://www.youtube.com/watch?v=mDsL4CwBEL4
That incident cut off ALL electrical power to the area around Grantham in Lincolnshire for about 5 or 6 hours whilst Western Power tried to re-route power around other routes, and the affected area stretched as far as Bourne