DweomerFall could be the result of a breakdown of the laws of magic. Like, say, with thermodynamics. Energy can neither be created or destroyed. Say the same is true of magical energy. The DweomerFall is a breakdown of that fundamental law on the 'or destroyed' side. Using magic (Arcane keyword powers, magic item properties, new rituals, magic item powers, etc), destroys magic instead of temporarily transforming it.
If you want to give us interesting decisions (an important part of any good game, IMO) relating to how we manage the DweomerFall, let us choose if we can accept instability in our powers or items. If we choose stability, we hasten the overall effects of the DweomerFall (sensitive arcane creatures or sites suffer). If we choose instability (we accept some unpredictable or lesser effects for our powers or items), we can slow the effects of the DweomerFall.
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I totally agree with Walking Dad that making arcane based PCs less and less effective out of balance with other PCs is unfun on it's face. Could it be made fun? Possibly. But I agree that it should affect all PCs similarly. I mean, I like the idea that magically-barren Harnithael's 'condition' could end up saving everyone, but I'm not sure something like that needs to be woven into the largely well-balanced core combat mechanics.
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Our PCs could have developed a way to - through implements/items/rituals, stave off the effects of the DweomerFall (or if they themselves did not develop these methods, they are road-testing them for the House Mages).
Just some thoughts.