NavList:

Frank,
You could be right, that in the late 1970s people thought GPS devices would never come down in price enough to be common accessories in civilian cars.  Maybe their size back then was also a factor. But I'm now thinking memory was the main problem. 
The first device I got (a Magellan GPS 2000 in 1996) still displayed only coordinates.  So did my next two (Garmins).  They were quite magical in their day because they so easily outperformed a sextant, even before Selective Availability was turned off.  But they weren't much use for getting from point A to point B in a car.
Now we have devices even smaller, running on 1 GB of RAM and 2 GB of fixed memory preloaded with a complete street map of one or more entire countries.  They have an animated color map display and a pleasant voice that tells you which lane to be in!  Features like that were utterly undreamt of until well after the 1970s.
Roger

Chester Kleczek, who at NAVAIR was the original sponsor of my Dad's Timation (for TIMe naviGATION) predecessor program to GPS, estimated circa 1970 that GPS (or Timation)  receivers would eventually come down to the cost of a color TV.  That's not a bad estimate for the time.