Do you think cutting the arm was enough to stop the effects of the Anchor?
Maybe they forgot it, but if it was that easy Solas would have mentioned it. So It could just delay the death of Inquisitor
It's been a while since I played it (game at release, dlc at that release) but I feel like Trespasser had a recurring theme of "Cutting your arm off should fix this" and it was mostly a matter of wanting to resolve the current situation before resorting to that. Again, could be wrong, but I'm almost positive they mentioned it a few times just to reinforce that that was likely to happen as a 'fix'