Planes have used inertial navigation for a long time. Adding GPS to that mix is relatively recent. GPS was never really intended to be used for navigation even though it can be and often is. There was no need for it to be particularly reliable when it was designed since the Soviets could easily kill it if they wanted to. It had a different purpose.
Can you elaborate on what you mean by "not intended for navigation"? GPS was intended as a replacement for TRANSIT and LORAN, which have navigation literally in their names. It's also the textbook example on Wikipedia's satellite navigation page, and I can't imagine how precise geolocation somehow wouldn't be useful for navigation.
Also, the general analysis after KAL007 was that GPS should be made publicly available specifically to aid civilian navigation. They even identify what we call RAIM today to make it suitable for eventual usage as a primary navigation system.
GPS was trivially attackable from inception, that was not a design requirement given its purpose. The purpose of GPS was to precisely measure the planet in peacetime. A unique core capability of the US military is extremely precise INS technology, the capabilities and precision of which are closely guarded secrets and under continuous R&D. The precision of INS is dependent on the precision of your world model. GPS allowed extremely precise world models to be built globally, which provided the data model INS needs to be maximally effective. In wartime the GPS system can be killed by any near-peer adversary, but the damage has already been done to the extent inertial targeting systems have a precise model of the world.
No US military system relies on GPS for navigation. Military navigation systems use GPS as an untrusted source for fine-tuning inertial navigation within tight error bounds. Against US military systems, successfully spoofing GPS might buy you several meters of deviation. This only affects the cheapest US guidance systems, since most weapons have active terminal guidance. It is worth noting that GPS corrections are being phased out in US weapon systems, purportedly due to improvements in INS tech that moot the value of GPS corrections.
The tl;dr: GPS was developed to build a precise model of the world in peacetime that could be fed to inertial targeting and navigation systems in the complete absence of GPS. In that, it has been a massive success. US military systems have never relied on GPS for anything important. Contrary to popular media, the US has never produced GPS-guided weapons, even the cheapest systems are INS at their core.
Civilian systems use GPS for navigation, against its design, because they can mostly ignore its trivial susceptibility to hostile actors.
Most of the basic attacks against GPS also worked against TRANSIT, LORAN, OMEGA, and others, which were clearly used for military navigation. I'm not seeing the design differentiation with the systems that were guiding ships and missiles.
All US military navigation systems use INS pervasively for primary navigation. You can attack GPS, LORAN, etc and it won't affect their operations. This has been axiomatic in the design of those systems for many decades. US military ships are no different, also using INS for primary navigation. As with every other US military system, they can conditionally accept fine-tuning inputs from GPS and other untrusted sources of navigation data because why not. They use a lot of open source data sources, but they don't need them or trust them.
