Re: Time
"Inertial nav is good for an 11 hour flight. So no need for sextants unless really long-haul."
Depending on your inertial nav hardware, you might well be right. But don't forget that a 4000-hour mean time between failures* doesn't seem so much when you have several thousand aircraft flying 4+ hours each; fortunately the equipment is normally far more reliable than the requirements state, but there is a reason aircraft (especially those carrying fare-paying self-loading cargo) prefer to fly with backup systems.
* I was involved in acceptance testing for IN gear in the mid 80s and that was the target at the time. I hope it has become more reliable since then, but then someone rolls out an update and...
Biting the hand that feeds IT
