NavList:

I feel that you're setting up scenarios that are too theoretical, too fantastical, and then considering a worst case scenario that's too extreme.

First, if all the GNSS satellites go down, your next stop is the nearest safe harbor because something is very wrong with the world! It's hard to imagine a case where you couldn't get to safe harbor in seven to ten days. The odds of such an extreme event are exceptionally low, of course. Should we even consider this sort of zombie apocalypse type of event? Preparedness is about planning for likely failures.>>

I do understand what you're saying but want to respond to part of this.

I disagree that a failure of GNSS means that a) something must be very wrong with the world and b) that the best way to respond to this is to head to the nearest harbor.  For example, being several days into the 25 day typical passage from Cocos Islands to the Marquesas, it would be difficult and probably unwise to return upwind with the idea that things would be better in one group of remote Pacific islands than another.  Especially if the root cause of the problem is that C&C for GLONASS crashed due to an error by an overconfident operator, and C&C for GPS crashed because of a Wannacry/stuxnet hybrid (and the Airforce won't pay enough bitcoin to some hacker in Romania to get their satellites back).

Maybe that's silly.  GLONASS isn't any more likely to crash due to an operator error than a nuclear reactor is to melt down due to an error by an overconfident operator.  And the Airforce is too smart to lose an important C&C system to a virus.