There is also inertial navigation, which has improved and become more cost-effective. I believe most large airliners now carry inertial systems which swing into action automatically when the GPS signal is lost

Yes, but the cheap UAV have much less mass, much worse intrinsic stability and much worse vibro-insulation than an airliner, or even a cruising missile.

Combined guidance systems can stay mostly functional when RC and satellite positioning are down, sure… but they are more expensive, which is why those vulnerable shortcuts are used at all.

And most of the rest has other vulnerabilities. I mean, if a laser gyroscope IS the only instrument necessary, you can’t fool it, but that’s not going to be the common situation for precise munitions. Flares are still a thing, and likewise you can use decoys to lead an anti-radar missile by the nose, etc.