Earlier quoted context omitted.
They can already perform accurate strikes without GPS with terrain following and INS so not sure this would actually help it that much.
My point wasn't about it being necessary. The point is what it may happen as part of complex engineering involving multiple components and large organizations. And/or by integrating consumer tech - the cheap consumer drones are actively used in the modern war and they may have such fallback mode which would be fine for civilian drone, until of course the drone gets a real warhead attached :)
For something slow moving like a quadcopter drone munition it might be viable but it seems like you'd not get good signal until it was too late to maneuver if it were in something fast moving like a guided bomb or cruise missile flying down.
Also the probability that power is still functioning long into a conflict is pretty low so it may only really be useful for early strikes.