I laugh at people using their phone for navigation, and this issue that it doesn't work in areas without cell service, just buttresses my opinion.
I remember being a passenger one time with a guy who had the phone in his lap and was always glancing down at it for the next directions. Scary ride. And what happens if you get a call right before you enter the mixing bowl in an unfamiliar city? Take the call and miss the directions?
I bought a new Garmin big screen a few months ago as my current 10 year old model was having issues in hot weather. It's got free updates for life like my old one and they come out every few months. I still like and use the old one when it's cooler since it's more compact. And I do have Google Maps on the phone but it's so clunky to use, I'd never have it as the only navi.