In the future, if we become over-reliant on GPS and it is suddenly turned off, how would the airline industry cope? Easily? With difficulty?
Quite easily, with tried and true stand-alone enroute navigation technology, INS.
Having been personally using INS for enroute navigation since the very late nineteen sixties/early nineteen seventies on 707 aircraft, it can be very accurate, for the enroute case.
The Litton units were/are more robust, the Carousel units more accurate, in my experience.
Plus/minus one nautical mile enroute navigation (over long enroute sectors)accuracy is possible, IF the specific units are maintained properly.