If I've read the Boeing publication correctly, the speed decayed from normal to Vref-40 in 30 secs. We don't know what distractions occured during those 30 secs.
This is not to excuse negligence if that is the case, but on the other hand, can any of us truly say that in our whole flying career we have never been distracted in such a way? For 30 secs?
I agree that a rad alt failure is a total non-event - I'm sure I've had loads of erroneous displays over the years.
Did the aircraft meet the stabilized approach criterion at 1000 ft? I think this crew "lost it" well before those 30 secs to Vref-40.