Unless one day the satellites all go off-line, we'll be fine.
Or the US turns dither back on
Or the GPS is jammed (the military can do this and have done it regularly in the past)
Or the aircraft GPS receiver goes U/S. This frequently occurs due to antenna failure.
Or the data card is corrupt
Or there is a RAIM blackout at the wrong time
Up until now, we have always regarded that you need a seperate redundant system. Why does GPS change that? Other than it makes AsA's life easier.