If that's the case, then why would he tell the recruits after the Joining there was plenty of time to let them know everything about the Wardens.That comment definitely implies Duncan expected to survive the battle.It seems to me Duncan was trying to have it both ways-let them know they might not survive, but also try to convince them they were going to win.
The thing that bothers me is there were no preparations made in the circumstance they would lose. No future Warden commander announced, nothing on the Warden secrets or any supplies of Archdemon blood or how the Joining is made, nothing on future recruitment plans, and all Wardens on the front lines.If Duncan thought the Wardens would lose, those preparations should have been made. More than anything, that lack of plans leads me to think he believed Cailan-it would be a glorious victory.
Like I said, I think he was buying time for the Wardens of Orlais, the Free Marches, and Weisshaupt to get into gear. I imagine he had already written Ferelden off as a lost cause once Loghain refused to allow the Orlesian reinforcements in. But given that they had already beaten back several waves of Darkspawn attacks, he probably thought there was more time before the big push came and that he would have time to instruct the new recruits. Who knows, perhaps there would have been had Loghain not deserted?
But not even Cailan, for all his bluster, believed they would win at Ostagar, Elric Maraigne (one of Cailan's bodyguards) says as much in Return to Ostagar. Ostagar was, at best, a holding action until more reinforcements could be brought in, but because it was a Blight, and not simply a Darkspawn incursion as people like Loghain assumed, eventually Ferelden's forces would have been depleted (or overrun) and foreign forces would have either had to be called in (despite Loghain's objections), or would have simply moved in once there were no more Fereldens left to stop them. Doesn't Riordan mention at one point that the Wardens and armies of Orlais are waiting at the border for Ferelden to fall so they can move in and deal with the Darkspawn without having to waste time and men fighting against Ferelden forces?
Regarding the Archdaemon blood and the joining, I rather get the impression that the Wardens reveal information on a strictly need-to-know basis (possibly based on duration of service or rank); so new recruits are probably never told the truth about the Wardens until their allegiance and commitment to the cause has been confirmed (ie. they have been fully indoctrinated). Besides which, if they were likely to die in the upcoming battle anyway, telling them all the Warden secrets might not have been the best use of the limited time he had left. It may have been as simple as him thinking "Either we will live long enough for me to tell them, or we won't; in which case it won't matter."
I don't have any concrete proof for my theory, it's based more on Duncan's pleas to Cailan to bring in more troops from Redcliffe and Orlais (and being knocked back) and his generally grim approach to the whole thing; he appears very much like a man who knows he is marching to the gallows and is doing the best he can in a no-win situation. His efforts to fight the Blight are being hamstrung by a complacent political leadership that does not really believe the threat is real and refuses to take the steps necessary to actually fight it (Remember that the rest of Thedas thinks the Darkspawn were defeated for good after the 4th Blight). All he can do is try to do as much damage to the Darkspawn as he can and hold them back for as long as possible, and then hope the rest of Wardens from other countries can finish the task.