Alternative navigation
Anyone still remember celestial navigation or dead reckoning? Know your stars and where they should be positioned relative to the track you're flying? Know your winds and airspeed and calculate (compensated) compass heading? Use your watch? This is the stuff that keeps you in proper stand-by mode and may prevent you from falling asleep on long (boring?) flights!
Should a pilot's brain not be prepared at all times to take over from a malfunctioning instrument and/or refuse a T/O when the instrument is vital
to exact position information, particularily where fanatic idiots take potshots at your aircraft?
And don't give me this "old fart" argument, that's been used too often.