Magic fading to extinction isn't actually the default setting of ArM5 - page 217 of the main book states:
"A common trope in stories involving a magical past is the decline of magic, so that eventually the magic goes away and leaves us with our contemporary world. The canonical background to Ars Magica assumes that, if this is happening, it is so slow that no one has yet noticed it. Magical auras give way to the Dominion in some places, but in others magic grows stronger."
Well, there was a reason for writing the word 'maybe' (you even quoted it, Salutor!).
But I think the whole House Bjornaer, many Merinitae and pretty much all magi in Normandy will be worried about the expansion of the Dominion and the diminishing of wild areas by 1220.
But that doesn't mean the end of magic; just the current, (maybe very long) slope. I dream of running a long range saga which would end with the Order having to wage war with the Dominion for the survival of Magic. Ah, times with Angel Eternal Oblivion spells! That would be huge.