You have to spend time and money teaching soldiers and sailors to use alternative methods, backup systems and work-arounds. This means you cut training for the system you'll most likely be relying on. This is why the Navy cut out celestial navigation instruction; it made little sense to the bean counters to spend the effort to spend hundreds of hours per navigator training for a skill that would probably never be used. They've learned since that was a false economy...

And its not just navigation, we have GPS systems guiding our bombs and missiles accurately to their targets, do we want to spend time and dollars training pilots and missilemen to use conventional techniques that will probably never be needed unless we go to war with an advanced technical power with the means to defeat our GPS advantage?

You must know yourself that even infantry needs compass and map reading skills that are hard to teach to the average grunt. Its cheaper to just teach them how to push a button on a GPS.

Radar did not eliminate ship collisions, and GPS did not stop vessels from running aground. In times of stress and confusion, systems, (especially their human components) tend to fail. And the most stressful and confusing of times is combat. Those seduced by the charms of high tech consistently overestimate their own ability to use it.