Here's a thought exercise. I think we can all agree that giving the orb to Cory was NOT GOOD.
What would have happened if Solas *hadn't* given the orb to Cory?
No explosion, right? Right. Would the mages and templars have stopped their war? Maybe. If yes, no Inquisition. If no, Inquisition gets formed with Divine Justinia backing it to stop the mages and templars - at least so Cassandra implies.
Cory doesn't just disappear. He's still planning to take over and make himself a god. He has the Venatori. He has the red lyrium. He probably even has the Grey Wardens. Most likely, he's STILL planning on assassinating Empress Celene and invading with a demon army. He doesn't NEED the orb for any of that. What does he need the orb for? Entering the Fade. What does he do without the orb? Looks for alternate ways to get there.
Without the explosion at the Conclave, no one knows what Cory is up to. No one even knows Cory exists. Cory is pretty much free to do whatever because no one is trying to stop him.
Obviously a great deal of this is conjecture, which is why it's an exercise in thought.
So, what does everyone else think Cory would have done without the orb?
Exactly what he planned to do all along, except nobody would have even noticed because they were all too busy fighting each other. So, Solas, seeing what Cory is all about decides to give him the orb so it can be unlocked. This tells me that maybe Solas wasn't too concerned with Cory entering the Fade. Let's say Solas knows something we don't that means this isn't important to him. Then our character comes along and interrupts the ritual, causing the explosion, the breach, demons everywhere and killing a whole bunch of people. So, Solas decides to join the Inquisition, ostensibly to help seal the breach, but probably also to keep an eye on this idiot with the anchor on their hand who totally (screws up his plans) changes everything.
I think we can all agree, that Solas and his plans, are very complicated. Like Jon Snow, we know nothing. Like Cassandra, we know less than nothing. Who knows, maybe Solas' plan was a really good one and we bungled the whole thing.
Damnit, Bioware. I have been able to happily walk away from all of your previous games. Why can't I stop thinking about this one??