There's lots to the story that isnt the fault of Magellan or their data providers, starting with the fact that they ignored the GPS that told them to take US 93. And ID 51 isnt "a few miles to the west" of US 93, it's got to be a good hundred miles or more. It sounds like they had a paper map, which even the most undetailed of which would have told them the two roads are separated by wilderness areas, meaning there were probably no roads to get from one to the other. All of this before they got to the confusing crossroads. Had they stayed on 51 instead of trying to outsmart the GPS algorithm, they would have eventually hit I-84, arrived in Vegas a little late, and would laugh later about how they should listen to the GPS next time.
As another commenter pointed out, had they simply entered "destination='Las Vegas'" and blindly followed it (as the article ironically seems to argue against), they would have been fine.
EDIT: As for GPS options, the BMW-branded Garmin for my motorcycle has an "offroad" option. Because middle-aged men riding 600lb. "dirt bikes" like to pretend they're mighty adventurers. Better know what you're doing, because that thing will lead you down roads that aren't passable the majority of the year (or passable at all on an R1200GS, in some cases). What you propose isn't practical because one good rain and the road can go from "okay if you don't mind dings in your minivan" to "you brought a lifted Jeep, right?" very quickly. Or as a more recent example from my other comment in this thread, that road i bailed on this weekend needs snow tires and preferably 4wd right now due to snow. I'll bet in just a few weeks you can take a lowered Civic over the pass.
This line seems to suggest that the GPS wasn't ultimately at fault?
My GPS has options for avoiding tolls and the like, but I've not seen one giving the ability to define the capability of the car. That said, there is huge variability in quality of unpaved roads, especially when impacted by weather.