Still then a retcon mission to save some grey wardens. He got so lucky the HOF is badass... I can't praise it as a good decision.
I definitely understand people who hate Loghain and even those that believe his decision at Ostagar nothing but a treachery with post-factum added explanation (or even orchestrated failure at Isthal).
However, holding against him the fact that he didn't know how important a Grey Warden is in defeating the Archdemon? That's just silly. We can hold people accountable when they make decisions bad in the light of information they have OR if we believe they could've had more information but neglected it. But when GW keep it a great secret how killing an Archdemon works AND they don't even disclose information that would make their "it's a blight, there is an Archdemon" anything more than a hunch - how can you blame the general for not taking into account those facts that were hidden from him? If you wanna blame someone, blame Wardens who had to be so damn secretive that the Ferelden could burn just to preserve their secret!
If Duncan told Cailan and Loghain why he thinks it's a blight - perhaps the whole strategy would be different. Perhaps Loghain despite his paranoia would actually see the darkspawn for the threat they were. Most likely he would at the very least cut the deal with orlesian Wardens to come help (while still denying entrance any other forces) or maybe he would bring in Wardens from some other place (Antiva? Nevarra?).
But Loghain didn't know. Nobody knew. Even as Ferelden was waiting to be destroyed, with no Wardens that would give any chance of actually ending the Blight - the Wardens sent a SPY. Not anyone to go up to Loghain and disclose their painful secret in hope that it could let them save the kingdom, no. Secrecy was more important, let the Ferelden burn if it must.
Sorry, I understand Wardens, I like them (a bit). But if you want to blame Loghain for not taking into account crucial piece of information PERHAPS it would be a bit more fair to rather blame those who hid said information from him, wouldn't you say?