NavList:

I don't believe GNSS is immortal.  I don't think it takes very much imagination to think of plausible causes of GNSS failure serious enough and lengthy enough to disrupt navigation.  Disrupt it for long enough that someone with a sextant who knows how to use it would be a godsend.  Examples: chain-reaction collisions of satellites in geosynchronous orbit rendering the entire orbit unusuable.  Unanticipated solar noise.  Enemy action against particular satellites.  Enemy action (hacking) of command and control systems.  Deliberate administrative decisions to degrade or shut down service, possibly on a regional basis, by system operators.  I don't think these outcomes are likely -- I'm not a kook and I don't buy lottery tickets.  I just think that the likelihood of these outcomes is difficult to assess and that the prudent mariner should be prepared.