I remember reading it in Lore somewhere. Keep in mind Ferelden is looked down upon and essentially considered barbaric, which would likely add to the skepticism some may have on the legitimacy of the Fifth Blight.
Um...you might want to go back and check up on that one, Challenge. The only lore entry I recall that mentioned that there are people that aren't sure if the Fifth Blight was a Blight is the one of the Sister Petrine entries. She mentions that some of her peers are uncertain if the Fifth Blight can be considered a Blight, but thats it. The idea that whole countries don't believe the Fifth Blight was actually a Blight simply isn't there.
So, yes, X Equestis' comparison is fairly valid. The overwhelming majority of people accept that Ferelden suffered a true Blight.
Even if they didn't it doesn't explain why the Warden. I doubt s/he's the only one who's ever done anything useful that is still alive.
Such as? Cassandra doesn't want the job, Hawke might be the Champion of Ferelden, but foiling a insurrection by stranded Qunari doesn't even remotely compare to ending a Blight after only one year, and in the middle of a brewing civil war to boot. They don't just need someone competent, they need a rallying point for the Inquisition's followers, and a figure of respect to give their enemies pause.
Frankly, the Hero of Ferelden fits that bill perfectly, even if they're a an evil bastard. The Inquisition under Justinia wasn't going to be an independent entity in the first place, it was going to be part of the Chantry, and thus subject to the Divine's authority.
EDIT: Found it. Study of the Fifth Blight, Vol 1