The problem about backing Solas' horrific plan is that there is no guarantee that it will not end as it did before; with powerful mage tyrants ruling the world and speeding it to its destruction, which he claims is the reason the evanuris had to be stopped before. Essentially what both worlds have shown, with and without the Veil, is that those who are the strongest and most powerful tend to end up lording over those who are not so powerful, spirits included.
Tevinter is a wonderful case in point since they demonstrate that after the fall of the elven tyrants, another mage empire emerged from its corpse, using what it could scavenge to advance its own agenda. The way to make a better world is to try and change people's thinking rather than simply the magical environment. Strangely enough, if the teaching outlined in the Chant is genuine, then that is what Andraste was trying to do:
"Magic was made to serve man, never to rule over him. Foul and corrupt are they who have taken his gift and used it against his children........Those who bring harm without provocation to the least of His children are hated and accursed by the Maker," etc. Sadly those who claim to follow the Chant only pay lip service to these ideals.
There is an interesting exchange in Asunder where Justinia is arguing with Lord Seeker Lambert about improving the lot of mages. Lambert argues thus: "And what price would you have us pay for such idealism, Most Holy?" Her reply: "Idealism is our stock-in-trade, Lambert. A religion without ideals is tyranny."
This is the problem with Solas' plan. Even if you could justify killing millions of innocent souls to put Thedas back the way it was (which I don't), it will mean nothing without some sort of leadership and moral framework to govern by. I'm also rather worried by his rather vague assurance that he "has plans" to deal with the freed evanruis, when he admits that previously banishing them was the least worse option.
As for who will survive, well if it really does burn "in the fires of chaos" then I doubt anyone will be left standing, except spirits. I have a nasty suspicion that this is actually part of Solas' solution. Mind you, from Cole's words it would seem that the spirits aren't enthusiastic about the Veil coming down either. The trouble is Solas' twin companions of "Wisdom" and "Purpose" have now been reduced to just purpose. Mind you, apparently wisdom was present when he had his original idea of giving his orb to Corypheus, so it would seem that "Purpose" has always had the dominant role in this thinking.