Satellite systems fail, there've been quite a few threads about problems with Europe's Galileo system, for example. There can be solar and other astronomic events which cause satellites to become unavailable. The GPS system became available to the public because of political decisions, it could be come unavailable for those reasons as well.

Making the entire country dependent on GPS limits the options of the military organizations that established it in the first place, and compromises their mission to provide navigation to troops in the field. (in my opinion)

The vulnerability of satellites to deliberate attack should not place the entire nation in a position of vulnerability as well.

A ground based system in friendly territory, with a large number of separate sites, provides a redundancy in numbers that you just can't get from a few tens of satellites.

There should also be ongoing research into improving celestial and inertial navigation systems as yet another backup.

A long range system similar to the Omega system should also be deployed.