Having just returned from Moab where I used a full suspension mtb (Trek EX7 Fuel) on various routes, including Slickrock Trail, I'd say a rigid fat bike does not replace a FS mtb - not even close. Even with my fat tires (Spesh fatboy with 4.6 Ground Controls) I would have been beaten up and far more selective picking lines. On my return it was a bit of a wake up call to again do downhill on my FB.
Doesn't mean I don't love my FB, but it isn't equal to a dedicated mtb.