In Exile
No, we didn't see the results of his plan at all. We saw the area around Castle Redcliffe, and that in itself was in flux due to Alexius' constant writing and rewriting of portions of history. Corypheus gains nothing by completely destroying the world, but it would be foolish to imagine that such a conquest wouldn't cause devastation. But cities can be rebuilt, and civilization finds a way to survive. Change is never a gentle thing. But between choosing Corypheus who has sentience and a desire to rule, not destroy the world and the Blight, which will indiscriminately poison, corrupt and kill every living thing *in the whole world*, I'll take the former.
Don't kid yourself - the situation with the Blight is *dire*. The Dwarfs are on their last legs (or do you think after the loss of their whole kingdom, 2 cities miles and miles apart will hold out forever?) and the surface population have once again lost all interest after the surface attack finished. If something isn't done about this, then nobody will survive, under a harsh regime or no. And since Corypheus has demonstrated some ability to control the taint, he may be able to exert some control over the darkspawn. Its more sucess than anyone else has had with the taint (including the Wardens, although Avernus made some progress admittedly, before the majority of players shut him down), The Blight is a menace that threatens global extinction, because it poisons the land. So even if people do survive the attacks and find safe havens to gather, the world will be devastated and (effectively) irradiated so it becomes useless.
Are we supposed to wait for the people of Thedas to come together and deal with this problem? Given that they have for centuries and centuries refused to admit the danger? Despite the wars with Dumat and Zazikel lasting centuries themselves? And what state are either the Dwarfs or Elves in these days, to render any kind of meaningful aid? Both races are well past their golden days, and both of Thedas' super powers (Orlais and Tevinter) are in a state of gradual decline (by their own admitssion). So unless you want the Qunari to take over and convert everyone to the Qun (because they at least would see that something has to be done definitively about the Blight), then I'll go ahead and cast my vote for Cory.
His regime would be hard, but life *is* hard. Wars happen all the time in Thedas (Tevinter and the Qunari are in a constant state of war in fact). Is it really going to matter to the people killed that it happened by demons instead of other invading nations - they'd still be just as dead either way. In an ideal world, there would be no war, but that isn't the reality of Thedas (or our own world quite frankly). War happens, people die, so with that in mind someone has to work out which of these blood soaked paths will leave some kind of world once the dust finally settles.
If the darkspawn are left alone, they will continue to breed, and eventually they will push to the surface again. All it takes is for it to happen in the wrong place at the wrong time, and its good night Vienna. So pretending the Blight doesn't exist and will just sort itself out is no plan. Thedas has no future if things continue as they are. And with the Grey Wardens mired in yet abother scandal and apparently heading for civil war in their own ranks, I really don't see the people of Thedas suddenly getting serious about this.