GPS, for all the launch and support costs, has become so much more than just a navigation source. Fields are plowed and harvested using differential GPS, heavy equipment can build roads with automatic positioning, and the ability to track an item in a logistics train down to the meter has enabled "Just in time" logistics to flourish. Not to mention all the mobile technologies that use GPS.
Loran is ok... but its just an antiquated navigation scheme. It's like saying "let's bring back A-N beacons!" If we felt we needed a back-up navigational network, a mesh network using cell towers would be cheaper and just as effective at this point.