The crew weren't promised several redundant levels of safety. That was the promise made to the passengers, and one of the most important safety barriers was the crew itself.
The other members of the crew should have objected, but they were not given much a chance.
As for these NZALPA 'believers' who say that the same thing would have happened to them, this is what a C141 pilot did about 45 minutes after Capt Collins: "At the time we we navigating entirely by INS. We maintained 16000 feet until McMurdo picked as up on radar; as I remember, this was at about 38 miles."
It wasn't rocket science and it wasn't complicated, either today, or in 1979, or in 1949. When you're at 16000 feet, you won't hit a 13000 foot hill.