Think of it this way. A person can live a normal life with 1 arm, 1 leg or even 1 eye or ear. Is it ideal? Probably not. If you lose your other arm, leg, eye/ear it's going to cause many more problems than just losing 1.
Losing any backups in aviation is a less-than-ideal situation. That was the only point I was trying to make. OF COURSE pilots can operate a plane without GPS, my god I'd hope they could. I feel like that kind of goes without saying.