Having had my say in a few of the Loghain threads on the forum it made me wonder if Loghain were not thinking of making his own grey wardens with loyalties to the crown and himself.
Riordan says that to induct more grey wardens they need a pinch of blood from an Archdemon and Ferelden's stores are missing. Did Loghain/Howe plan to interrogate Riordan (torture is such an ugly word) to obtain the details of the Joining?
Paranoia, paranoia, everybody's coming to get me 
That's always been my belief. Howe had classified Warden documents, the Warden storehouses were raided and the Joining supplies confiscated, and Anora is knowledgeable about the fate the Joining has (which was a secret, as Jory demonstrated)... so it leads me to believe Loghain wanted to create his OWN Wardens eventually who would in fact be beholden to the nation of Ferelden and be useful in the coming battles.
This knowledge however, while it would enhance the argument for the Wardens' necessity, doesn't validate it right away. After all, it's still unknown to everyone that only a Warden can end the Blight, because of drama.
Naturally, by the time he discovers all of this, the Warden and Alistair are already his political enemies, plus he has understandable reasons for viewing them as Orlesian agents. So diplomacy with them isn't going to be an option, much less since the two of them aren't willing to really engage in such with Loghain.
The only thing that throws a kink in this is how Genitivi also knows of the effects of the Joining, by way of the Soldier's Peak codexes. As a result, the Wardens' "THIS IS A SECRET IT MUST STAY A SECRET" comes off as silly writing.
So yay for Bioware screwing up their own lore!
Besides, if Loghain wanted to create his own Grey Wardens, why would he leave them to die if they are the only ones who know how to prepare the Joining? At the very least, he could have captured one of the Wardens before the final battle at Ostagar and took them away to Denerim for interrogation.
The Wardens didn't prepare the Joining. Duncan says he's had the Circle Mages prepare it.
And as to the bolded, one... he didn't "leave them to die". They just died in battle. Two, he would've discovered all of this after they were dead. Remember, after Ostagar he heads up to Denerim to recruit the nobility to his banner to fight the Darkspawn. With the civil war the Bannorn starts, fueled by the antagonism between Teagan and Loghain, the Teyrn spends his time either out in the field or occasionally back at Denerim which serves as his base of operations.
Nevertheless, upon returning to Denerim after Ostagar he orders his men to seize what's in the Warden storehouse.
wardens are not superior to the next soldier.
Good sir or madame, I'd ask that you look towards the Wardens of Soldier's Peak. One hundred Wardens faced an entire army and held out for months before their supplies let out, and even then were putting up a helluva fight when the soldiers stormed the keep.
Wardens are indeed superior to your average soldier.