Sextants, charts, NDB's, LORAN -- all of the above.
There is too much dependence on just one nav technique, or process. Especially in a day when an anti-sat, EMP or cyber attack could bring it down.
Good luck bringing down every NDB, Vortac, and LORAN site in the country. Doable, sure, but it that actually happened there would be nowhere to land anyway.