I kind of was under the impression that Cory is in denial about the corruption of his body/possibly the world his action brought about. Which is why, as compared to the Architect, he does not do much pertaining to the unique idiosyncrasies of the Blight/Taint. He uses Red Lyrium mostly, and Fade centered control over the Taint. in DA:O we are told that the Taint is foreign to demons/spirits and so gives Wardens power over them and Darkspawn Mages have unlimited power they draw from the Taint and not the Fade. Yet, in DA:I we meet a demon who basically knows how to brainwash the Grey Wardens for Cory, even though after DA2 it was pretty clear Cory could do that subconsciously, although it might have something to do with the fact he was asleep that he could do that in Legacy. Still, it seemed like Cory was avoiding actually drawing on the abilities of his corruption.
But to me two of the big symbolic actions of Cory kind of prove he was in denial about his Tainted nature. The first is the goal of entering the Black City again. He says that what he found in the Black City was either silence and darkness (DA2) or dead whispers (DAI). He does not directly implicate his entering the Black City with his corruption. Why? because to do so would either indicate he was punished, like the Chantry said, or that he was deceived by forces/entities beyond his understanding and that he claimed exactly what lies in the Black City, the corruption of the world. Or, possibly the more frightening prospect for him, that he and his cohorts did claim the power of the gods, and that power is the darkness and the dead whispers, because the powers of the gods he is harnessing are fallible and sundered. Cory wants completeness. He wants closure. He wants it to be painfully obvious what the divine is and its meaning to Thedas. He wants his faith to be obvious. He probably suspects something is "wrong," something is broken and not working properly on the "divine" level and that he feels he has to fix it. He wants to "correct this blighted world." To me, this indicates that the orb has probably revealed a lot to Cory, maybe not to the level he understands it, but that he can recognize something is wrong. Operationally, Solas' and Cory's goals are the same, they were both going to enter the Fade physically and do something, in Cory's case to "claim the throne" and for Solas', to tear down the Veil. I suspect Cory's actions would have done something similar, as everyone is convinced he would destroy the world, and that is precisely what Solas tells us he himself will do. That's why Cory wants to go back to the Black City, because he suspects that this world is "blighted" that it is wrong and he has to fix it, correct it. Compare that to Solas who admits what he did to the world in making it like it is, with the Veil, was a mistake and that Thedosians are all shadows of what was.
The second symbolic gesture Cory did which indicates he was uncomfortable with his Blightedness was the creation of the Fake Archedemon. By doing this he essentially gave his gods the middle finger and created the parameters by which he could be killed, by disrupting his body hoping by killing a vessel of his soul. So basically he ruins one of his Taint super-powers, which is immortality because the power of a symbol is more important to him than actual powers like immortality. We know also that Dragons have some sort of symbolism for the Ancient Elves and in particular their "gods." To me, this again gets back at what Cory was all about: he senses/suspects some mistake occurred which disrupted ancient forces he perceives as Divine, and he recognizes an absence that he as a deeply spiritual man (he was a priest, after all) can't accept.
That to me is why Cory did not harness the power of the Taint. Because he is not necessarily searching for power, but really trying to figure out what happened and if it can be fixed to restore, if not his faith in the Old Gods, but the value of Faith in Thedas at all. If there are no gods, then Cory will validate faith by making himself a surrogate. We never see in the Dark Future what happens if Cory had gotten to the Black City but only what happens if he could not fabricate another Anchor and the devastation he sews searching for another way into the Fade. I suspect that if Cory had entered the Fade in the flesh with the Orb/Anchor, what would have happened would have been similar to what Solas would have done.
So to me, that is why Cory was not necessarily a wasted opportunity because he did not harness the power of the Darkspawn, as I feel there are plenty of other Magisters left who could still grant you your wish. Cory is the foil of the Inquisitor because they are both grappling with issues of faith and belief. Cory is essentially on an esoteric mission, not really on a mission of global conquest. Though I do agree that the plot of the game, what with him seeking to conquer the South makes him seem like he's really seeking to rule from the Black Throne in the Black CIty, but the Ancient Tevinter Imperium was all about unraveling ancient mysteries they could barely comprehend. They were seeking answers. Cory is upset because in his search for answers he found only doubts in his nation, in his gods, and in himself.
That this stuff was not made more obvious, and Cory never really was developed as a complicated character outside of a couple of Codex entries you only get if you side with the Templars, and one understated line at the end when he calls out for Dumat, is the wasted opportunity for me. I guarantee we'll have another Darkspawn threat, just Cory was always meant to be a foil for the real threat of DAI which was not the Darkspawn or the Taint, but Solas. Cory was always meant to emphasize the consequences of Solas' actions: the creation of an incomplete world that will always seek to destroy itself in search of completion.