I’m not very familiar with the technologies you linked. However, bear in mind the military operates at a scale that’s hard to fathom. Fielding new capabilities requires substantial level of effort when it’s military wide. Aside from that, many times they already have some tricks up their sleeves if push ever came to shove.
Anyways, what I linked to are the key components for inertial navigation systems. Not all of them together, but several options. The first is that chip-scale 'atomic clock' thing from microsemi, a very precise time source. Something like that would be needed in every case.
The others are different possible paths to precisely measure (speed of) movement and (change of) orientation. Based on those you could construct inertial navigation 'on the go' in small form factors. Not smartphone-like small at the moment, but close to.
Regarding the scale of fielding/roll-out, ask Apple, Honeywell, or someone like that...
Thinking about it, maybe that is why 'the others' play at shooting things in orbit because they allready have 'Mao's mighty map' available? Who knows? Whatever. The end of GPS/GNSS wouldn't necessarily have to be 'the end of navigation'.
[1] https://www.microsemi.com/product-directory/clocks-frequency... ,
[2] https://en.wikipedia.org/wiki/Ring_laser_gyroscope ,
[3] https://en.wikipedia.org/wiki/Fibre-optic_gyroscope ,
and other
[3] https://en.wikipedia.org/wiki/Microelectromechanical_systems
gizmos like
[4] https://en.wikipedia.org/wiki/Vibrating_structure_gyroscope#... ?
Especially when global
[5] https://en.wikipedia.org/wiki/Digital_elevation_model
fit multiple times on the equivalent of some micro-sd card nowadays?
Seems like something like this
[6] https://en.wikipedia.org/wiki/Synthetic_vision_system
wouldn't need to depend on GPS at all, maybe some sensor fusion for things like radar/infrared/whatever.