Optimistic-ish headcanon regarding Solas's motives and the veil. 
Regarding Solas expressing the desire to destroy the world, despite the fact that the very idea of it obviously causes him enormous distress and runs so contrary to everything we know about him... what if there truly is no other option?
Think of it this way. The veil has obviously caused a great deal of damage to the world. It blocks magic, which means that the magic in Thedas has steadily been dwindling and may eventually cease to exist entirely. (There was something somewhere mentioning that fewer mages are being born into Dalish clans. Mages as a whole are becoming rarer.) On the other hand we have the taint, which has been infesting the world at a steady rate. Magic is often described as a 'song'. The taint is also described as a corrupted song, and there's still the question of 'silence' and what that means. (Possibly the death of the world via the removal of all magic?) What if the taint is the direct result of the veil either a ) blocking magic, or b ) corrupting it as it passes through from the fade to the physical world? As the magic in Thedas weakens, the blight grows stronger.
The veil might even be the void, stretched thin and covering the world. The magic passing through is corrupted by whatever it is in there that sent Andruil bonkers. It could also explain why passing through the veil is so traumatic for demons. They're directly exposed to whatever poison dwells within the void. Shattering the veil a la Corypheus seemed to unleash blight over the whole world at once - whatever was contained within got out. Possibly, Solas has a less blighty but no less catastrophic option for getting rid of it.
It could be that lifting/destroying the veil is the only way to eliminate the blight at it's source. Such a connection would also explain why Solas despises the Grey Wardens so much; he knows more about the blight than they do, and their actions may actually be making it worse. If the true source of the blight isn't stopped, eventually it will consume the world, leading to death and ultimate silence. Solas may regard destroying the world as being the only way to save it. (Conveniently restoring the Elves in the process.) However he knows that the act required to save the world will devastate the mortal races, hence his pain and turmoil over what he must do. (Also compounded with the fact that he knows he wants the old world back at the expense of the current one.)
Basically, I think there's much more here at stake than Old vs New and Elvhen vs Everyone Else. MUCH more.
This is an amazing post, and is almost exactly what I think too
Hopefully throughout the next game, we'll learn of the consequences of Solas NOT going through with his plan (now we know of some of the consequences of what will happen if he succeeds), and the choice of whether to try to stop him or not becomes a lot murkier.
Personally, I don't think we'll be able to stop him. I think the veil is coming down regardless (even if we stop Solas, someone else will step out of the shadows to finish what he started), but I think we might be able to 'steer him down a gentler path', allowing some of the past to be restored while preserving the present.
Ugh, such a fantastic character. All the kudos to Weekes, and to the guy who does his cinematics (amazing facial expressions) and to Gareth David Lloyd. Between the three of them, they have created something truly special.