Again, it's common knowledge Grey Wardens are needed to end Blights. Even Cailan knew this. If you want to roleplay someone skeptical, fine, but don't act like everyone else in Thedas ignores history.
Look into the history books and tell me "why" only Grey Wardens can end a blight.
You can look into history and see Grey Wardens "have" ended blights, and they make the claim that only a Grey Warden can, and that Grey Wardens are needed. But if you ask them and are not a member of the order on "why" being a Grey Warden is so special that only they can end the blight, they're remarkably tight-lipped.
Not even Alistair, someone who was a Warden for six months before our Warden joined, actually knew "why," and asks Riordan if there is actually more to it than bashing the archdemon over the head.
Anyone who looks at history in Thedas can see that Grey Wardens have ended the blights, and anyone who knows the wardens know they say that only a Warden can end a blight. But none of that tells you "why" only a Grey Warden can.
Without knowing the secret, a skeptical commander can look at the wardens and go, "So your order defeated blights four times in the past. Great, so how did they do it?"
Then the warden would go "I can't tell you."
Then the commander can, and likely would go, "Why should I believe you?"
Warden goes, "Because Wardens have ended blights in the past?"
Commander goes, "How?"
Warden goes, "It's a secret."
Then the commander bangs his head against the wall, remembers the wardens allied with Orlais and nearly got his best friend and king killed, and worked with darkspawn to infect all the surface and turn them into blighted beings, all within the last 20 years, and suddenly the wardens don't seem so honorable or needed.