So, if I understand it right:
- Solas wants to destroy the Veil again and thus making the Fade and the Physical world one and the same, again.
- The world will die (?) if he tears down the Veil.
- The Rifts are doorways between the Fade and the Physical world.
- The Breach was a huge rift and without interference would have grown until it swallowed the world.
This brings me to a few questions, maybe some of you can answer.
- If the world will die, wouldn't it kill all the elves all the same (and if not, wouldn't the humans, qunari (race) and dwarves survive as well)?
- Did the Dwarves already exist before the creation of the Veil? If so, what will happen to them and their Titans if the Veil gets destroyed?
- And the most pressing question; If the breach had swallowed the world if not opposed, why did Solas want to stop it? Since it would mean the Fade and the Physical world would be merged? Or wouldn't that have happened?
I am guessing I misunderstood or misread some of the lore, but if someone could clarify this, I would be thankful 
I think some of your confusion is coming thinking that the rifts are doorways - they're not, really, or at least not ones that anything but things of the fade (demons/spirits) can get through.
The orb's power allowed the Inquisitor (and friends) to walk in a certain part of the fade - it was Fen Harel's orb, so it allowed the wielder to negate what Fen Harel did: making it so that beings of the physical world could not physically be in the fade. As the codexes hint, the veil isn't really a barrier as such - it's more of a fundamental redefinition of the relationship between the fade and the world brought about, we now know, by Solas.
So when the breach emerged, it was very much Solas' priority to stop it - he couldn't enter the fade himelf because he didn't wield the power of the orb: the inquisitor did, albeit very imperfectly. He certainly didn't want to go up in flames while Corypheus tried to establish himself as a God, which as we know from In Hushed Whispers looks like it results in GAME OVER for just about everything. Interestingly, he and Coryfish' plans were almost identical - physically enter the fade, journey to arrive in some place (the black city, possibly arlathan?) and change the entire nature of reality to suit their respective purposes (restoration of Arlathan/Godhood).
Sorry if that was a little confusingly stated, hope it helped!