It doesn't matter if one satellite goes. Most of the time one is receiving as many sats as the receiver is capable of concurrently, 8 for mine.
Playing with the Eurocontrol website MJ gives shows just how good the reception is. Even when there is RAIM outage, this is not relevant to en route navigation. It's relevant only to GPS approaches, but
a) we don't have them in the UK
b) where they exist in Europe, they are merely supplemental to some other approach, and one has to carry all the kit anyway, according to the IFR/CAS requirements in various airspaces
c) in the UK anyway, it will be for ever before we get The One Really Useful Thing (GPS approaches into airfields that don't have any IAP) largely because of the cost (CAA requires full ATC, etc)
Galileo won't be any different; it can't possibly be. The physical reality is still just X sats whizzing around up there. Any guarantees of availability are just empty words from European politicans who are so superior to American ones
Well, they are not quite empty words, because they will make GPS approach legality dependent on paying for the "higher integrity" signal