Let's not forget that early detection is key! Duncan recognized the Blight for what it really is before even the rest of the world realized that there was a Blight. To this day, based on various codex entries, people (mostly outside of Ferelden) believe that the so-called "Fifth Blight" wasn't really a Blight.
Despite keeping the old treaties safe, and rescuing the Warden and Alistair from the Tower of Ishal, and let Morrigan accompany the party. I don't see her doing anything else that benefits the war effort to stop the Blight, but yeah...without her it could be worse.
I'm just gonna assume that she has some powers of precognition (she did tell Maric that there will be a Blight in Ferelden), and knew that the Warden and Alistair is all that she needed to save from Ostagar to stop the Blight.
I lose all of my respect for Vivienne when she says the Wardens are nothing more than a relic of the past. Seriously, do I have to throw her to the darkspawn so she can see how important the Wardens are. Do we really need darkspawn to roam the surface to remind us that they are always a threat to all, and without the Wardens...Thedas is doomed.
I'm sorry, the EO?
Same with Solas. For a guy who loves history and historic battles so much, he sure is quick to dismiss the Gray Wardens as whole. There are still 2 potential Blights in the future, and that is assuming that only the Old Gods can become Archdemons, and that those dragons were indeed Old Gods. For all we know, any high dragon of sufficient power can become an Archdemon.
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To be fair, Gray Wardens didn't exist before the First Blight. So, even if the order were to die out, and a new Blight appears, I'm pretty sure that some clever mage or knowledgeable historian can conjure up the Joining Ritual all over again.