It's not nonsense you are spouting, at the contrary. It is so much common sense that even "the powers that be" thought along the same lines. They even managed to make something workable out of it, it is called "satellite navigation". (also known as "GPS" to those unaware of other satellite constellations, Glonass, Galileo, &c)
if that backup isn't NDB/ADF, what should it be
For en-route navigation, I always understood EASA intended a network of DME stations, whose data could/would/should be triangulated into location/heading/speed information just like one gets from GNSS. But I seem to remember this idea has been shelved, and am not aware of any alternative plan. Still less information about any plan B against GNSS failing on instrument approaches. But was there any plan B on any NDB approach if/when the NDB should fail?