In addition to several of the pros you listed, I'd add a few more:
- Stupid-simple interface. I was a Google Maps user for a while, and I was not a fan of various popups (from "Welcome to <state name>" to "Is this speed trap still here?"). I'm piloting a several-thousand-pound hunk of metal at highway speeds, do NOT interrupt me!
- Unchanging interface. I'd get used to the way Google Maps behaved, and then an update would change something and break muscle memory. Whereas my 2015 Garmin isn't getting updated with unnecessary animations and pointless button re-arranging. When updating maps, the firmware update is a separate download. Since it's not Internet-connected, I don't care if I'm running "outdated" software. As long as it still gives directions, that's all I want.
- It's not my phone. I don't want my text messages coming up across the top of the screen while I'm trying to read the next turn on my route. I don't want anything on the screen that's not directly related to navigation.
It's a great example of KISS in a hardware product. It does GPS navigation. It is literally incapable of distracting you with anything else. I consider that a killer feature.