If the world was destined to end, the Canticle wouldn't have to reform and change itself each time it is defeated; true, it seems like a march to the end with no hope in sight, but the fact that it can be averted in whole or part means it is not predestined, but only possible - for that reason the question is somewhat flawed, but nevertheless we can fight the Canticle and struggle on to the bitter end because it can be temporarily prevented - better to buy oneself a moment of calm for the world than to accept the end prematurely.
Does that answer your question (in one hell of a run-on sentence)?