1. I don't want to create conflict with the Wardens. I doubt the First Warden would be happy with me exiling a couple hundred of his Wardens out of Orlais. The Codex says the First Warden is more concerned with politics than guarding humanity, so taking his influence out of Orlais is going to ****** him off.
2. If some Blight starts in Orlais or somewhere in that area, I would want to have Wardens standing guard. The next Blight could happen a decade or a century from now. By that time the leadership in Orlais and in the Orlesian Wardens would change. These same people who fell into corruption would not remain Wardens forever. Eventually better recruits, better Wardens will take their place.
3. I need more bodies in the army vs army that happened...somewhere, sometime.
4. If I ****** off the Wardens, I was afraid I might cross paths with the Hero of Ferelden or Alistair. And I'm sure both those guys will be pissed at me. And honestly, I like HoF a hell of a lot more than my Inquisitor.
Lastly, I knew I was going to kick Corypheus' ass by the time all this is over even before I saw the ending. There was absolutely no way Corypheus was going to win, so I didn't need to take into account corruption in the long term.