Gents
Everybody is correct in some form or another, basically it is fail operational so if you get redundacy on finals you can go around or land depending on the downgrade.It is a certification issue for the relevant authorities. At the end of the day you can autoland CAT 3 NO DH with CAT 3 Single.
The a/c only knows the ILS and the course and Flare mode, it will continue to land whatever the limits, do not forget only YOU know the RVR unless Airbus have made the a/c that clever.
Imagine you are low on fuel or some emergency!! What you going to do put it on final and autoland whatever the wx or DH is. All be it NOT legal!!!
Just my small input, so please no bunfight!!
elac2