Definitely need a backup, but as usual, simple answers won’t satisfactorily resolve the problem. As well as being inherently low rate, less accurate and requiring a fairly extensive worldwide network of easily targeted ground stations, LORAN has the same weakness to jamming as GPS, plus that jamming & spoofing can be done from a distance. Celestial and inertial both have their own weaknesses, so the real answer will be a complementary meld of multiple techniques.