That's a very good point. No Cyradis, no choice, although presumably if something were to happen to her during the journey, the Seers of Kell would have sent someone else in her place (being able to see the future and all.)
I'm not sure whether the two prophecies would have continued to struggle after that, though. It was coming to the end of their 'game', and the light that signified the moment of choice would have gone. Wouldn't that mean an end to the meetings between the Child of Light and the Child of Dark?
Thinking about it, I wonder what would have happened if Cyradis had not been able to make the choice, if time had run out for her to choose, as it were. With the Sardion and the Orb in the same place, surely there would be some interesting consequences.