When combined it is very hard to break the position fix. I expect the issue with the system is it's a military tech. with all it's that'll associated legal problems. After all. it'd bypass most the protections built into GPS modules.
Good luck finding a pilot trained on doing this, or an aircraft with high enough window cutouts that allow for the use of a sextant. Old Boeings had them [1], but no modern aircraft does.
I think even among navies you'll be hard pressed to find a navigator who still can do astronavigation. IIRC even the US Navy discontinued astronavigation, but recently-ish reintroduced training for it again.
[1] https://virginflightdeck.blogspot.com/2010_09_01_archive.htm...
Astro-inertial guidance is used for example in submarine-launched ballistic missiles.
"Astro-inertial guidance, or stellar-inertial guidance, is a sensor fusion-information fusion of inertial guidance and celestial navigation. It is usually employed on submarine-launched ballistic missiles. Unlike silo-based intercontinental ballistic missiles, whose launch point does not move and thus can serve as a reference, SLBMs are launched from moving submarines, which complicates the necessary navigational calculations and increases circular error probable. Stellar-inertial guidance is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfect instrument calibration."
Not necessary. ICBMs have been able to do this autonomously for a very long time [0]