If you're going to insult Corypheus, at least spell his name right.
Corypheus is not stupid and I say is not, because his spirit is in the Fade. He isn't destroyed.
His plan was genius. Kill the Divine at the summit crushing the faith of Andrastians. If the Maker will not save his most faithful especially the Divine, then either he has turned his back on the people or he doesn't exist. Use the Orb of Fen'Harel to create the anchor, giving him the ability to physically enter the Fade so he can return to the now Black City and claim the throne as his own. From there he can influence the minds of all who dream. Use the Grey Wardens to create an army of demons. He can't control demons but he can control the Wardens via the taint. With the demons bound to the Wardens, he can now control them and unleash them on Orlais. With the faith of the Andrastians shattered and demons everywhere destroying and killing everyone, all who dream will see and hear a "god" that will answer their prayers for help. Corypheus will answer the prayers of the people and destroy his demon army, "saving" them all. They will see him as a god worth worshipping.
Good plan, but it was spoiled by chance or divine intervention via Andraste and the Maker.