Even with failsafe, a full radio failure can continue flight for lots more than 5 or 10 seconds. I've had it happen twice, once due to a battery that ejected itself from the aircraft and another when the battery failed due to a bad charger and me not double checking the charge. Yep, both errors on my part.

One time I was in a big huge split S when the battery departed company of the aircraft, the other I was in a loop when the battery died in spite of having a fresh charge. Both had the same result. We watched while the airplane was "making lazy circles in the sky!" They looped and looped and looped, getting a little lower on each pass while drifting down wind. They did eventually contact terra firma but both made for long walks to retrieve the remains.

Unfortunately, I don't know of any sort of kill switch that is going to work after the receiver has lost power. Otherwise, failsafe is your best friend.

Dave