The short of it is that Dhaos, the villain bent on world-wide destruction, is actually doing it all to save his own dying world. It's a bittersweet ending in that regard.
I don't think we know that's the original plan. Bioware saying DA:I was planned to be twice as large could mean more content for the same end. But I agree with the idea that Corypheus was not fleshed out because Solas was the conceptual endgame.
True we don't know and I'm just guessing. I think Gaider did mention that they were originally thinking of more of a Wolf Hunt as the end of the base game rather than as the final DLC, which makes me think there was going to be a more involved Solas-as-antagonist element within the game.
Plus I'm wondering whether Corypheus was intended to be the final villain in Exalted March, which would make more sense to have a DLC enemy be resolved in another DLC for the same game rather than transfer to a new game. And since Cory's plot involved eluvians and the nature of the gods, we could have an introduction to Solas at the end of Exalted Marches, Cory opens the orb and somehow has a permanent death rather than regenerate, the destruction of the Conclave ends the DLC and then Solas is still a companion in DAI because the orb was destroyed in the explosion and he needs the Inquisition to help find other ancient elven sources of power.
Something like that...
The other thing I want to get out of the way is those epic plots make a world small real quick. DA has potential to be like D&D with decades worth of stories. Maybe (big maybe). But not if they blow so much away with drastic events.
Same goes for Mass Effect. That's why I'm weary of this new one.
I think Bio had the concept for a finite number of games for DA. The series is named after a century in the world, after all, and while that wouldn't preclude any game to occur after 9:99, it seems like the intention is that this century will bring about great change for the entire world. Maybe it will be lowering the Veil and changing the nature of magic in the mundane world. Maybe the old dragons will awaken and the titans will awaken and the Enuvaris will escape. But it never felt like the setting was intended to be an indefinite open location for adventures like the Forgotten Realms.