NavList:

>>Please do so identify those scenarios (for GPS failure).

One scenario is that the recreational sailor has a failure to appreciate the nature of an effective Faraday cage. 

The proper way to protect a handheld GPS unit from a lightning-generated electromagnetic pulse is to surround it with an insulator (e.g. inside a sealed ziplock bag or plastic case) and then surround it with the conductor (e.g. wrap in aluminum foil). 

This particular sailor at https://forums.sailboatowners.com/index.php?threads/faraday-cage.146943/ got it wrong:  he had electronics-conductor-insulator rather than electronics-insulator-conductor.

I had a GPS wrapped in tin foil, inside a Pelican Case stored in a Ziplock bag with desicant and it still got fried. 

This failure to understand the nature of an effective Faraday cage - which seems to pop up repeatedly on blue water sailing forums - has definitely led to instances where all the electronics connected to the boat's electrical system got fried by a lightning bolt, as did all the hand-held GPS units.  And because of their initial misunderstanding, they come out of their experience of a lightning strike no wiser than before it happened.

I have an acquaintance who sailed single-handed around the world in her 60s.  She did not carry a sextant, but rather had 5 hand-held GPS units to back up her main GPS.  But because, as I discovered, she did not understand how to protect against EMP, if she HAD experienced a lightning strike, she might well have found that she had a single event that caused a catastrophic, simultaneous failure of every GPS on board.

The best back up to a GPS may indeed be another GPS...but only if you have a correct understanding of EMP and Faraday cages.

It is impossible to know how many of the 40 to 60 sailboats a year that disappear without a trace do so after a catastrophic loss of GPS navigational ability.  It could range from "quite a few" to "none".

But one can at least imagine a scenario where -- no matter how many handheld units one has aboard as backups -- a GPS only approach to navigation leaves you with something very like a single-point-of-failure for a mission-critical capability required for making a landfall.

Bob