I dunno, I kinda feel like... like Cory was only a threat because the writers made him one. Like, he comes to power, because Solas makes a mistake and hands him the Orb. He gets his mages, because Fiona panics and throws in the Vints. He gets his demon army, because the Wardens freak out and try to suicide run the archdemons by slaughtering their own men. He gets Celene assassinated, because Florienne's a cackling one-note villain who's personality can best be summed up as "evil."
LOL, but that's the thing - for all his faults, Corypheus has brilliantly played the weaknesses of the system to build himself an army (and for all we know, he built that army probably long before he even got the orb, or an idea what to do with the orb). Which is exactly my point.
He's a really good schemer, but this is where most of his power resides (he was defeated by Hawke in Legacy, wasn't he? Without the orb he may still be unkillable, but he is defeatable in the fight). Meddle with his scheming and you start unraveling his plans. Even Solas tells disliked Inky that they shouldn't thank him, since without his our Corypheus would be less of a threat - but he'd still be a threat. Venatori, for all we know, have been working with him for years before Conclave even exploded.
It's like the Inquisition's able to beat him, not because they're just that good, but because they're half-way competent and don't drive themselves off of cliffs as soon as something goes wrong - and it's great that we were able to surmount that problem, but at the same time, it's like bragging about winning a race when all your opponents were snails and one pile of rocks. Sometimes it successfully came off as "The real issue is how these people are torn apart and unable to challenge the real threats." But most of the time it came off as "These people are helping Cory because we're the writers and we say so no questions."
So I find it kinda hard to fault Solas for being an idiot with Corypheus when nearly everyone else was, too.
...Jeez, I am being super critical today...
Well, that's the thing - we're building the power by at least attempting to fix the broken system, because that's where real issue lies. Do note that nobody was really prepared to do anything with the Breach, or Cory, or the conflicts until Inquisition came.
Orlais? Embroiled in a civil war. Chantry? Unable to deal with mage-templar conflict exploding all over the place.. only to literally explode at the Conclave. Free Marches - hardly ever united against anything. Ferelden? Still dealing with effects of the Blight. Nevarra? Apparently not interested with much other than their internal affairs. Wardens - well, there's been a whole slew of issues with the Wardens ever since DAO, aside from Nightmare playing on their fears now. The Qun - more interested with leaving problems like Blight or Breach to someone else and later exploiting weakened lands to conquer them. Tevinter - deeply weakened, and at the cusp of both civil unrest and proper invasion from Par Vollen.
The fact that Inquisition even managed to accomplish what it did in such critically broken world is a miracle in itself :/ TBH, I can't not understand Solas wanting to push the reset button - I myself am tempted, the more I think about it
...
Anyway - the critical weakness of most other countries or organizations we know of is also the same reason to believe that Inquisition is the only organization capable of going against Solas in DA4 - it truly is the only one of the few powers left standing, even if smaller after Trespasser.