You can say, won't bringing down the Veil let the evanuris come back? His reply "I have plans." Hmm, like his grand plan to put up the Veil in the first place, apparently not realising what it would do to everyone else apart from the evanuris, and his plan to give the orb to Corypheus thinking it would blow him up and he could reclaim it. Solas has plans but with a sort of tunnel vision that precludes him from even considering that his plan might go wrong until it does. He thinks if he tears down the Veil he can just restore everything like it was before. But he didn't create what he destroyed, others did. So what makes him think his wretched plan will work? That presumably is the bit he doesn't want to tell you, when you ask why it is necessary to destroy our world to restore his.
His name is very fitting. He is seems humble but he is not and thinks he has the right to destroy our world and the gall to say he is not a monster in doing so, just because he made sure we had a few years of peace before hand, like giving the condemned man a last meal.
The thing is you can look at the ruling structures in Thedas and say nothing lost. I wasn't entirely against the Qunari plan to remove the leaders but felt the ordinary working people might not accept the Qun as easily as they thought. But Solas isn't just changing who is in charge. He is destroying everything; men, women, children, animals, birds, plants - everything will be destroyed in the fires of chaos. One of the early trailers for DAI went around showing us all the lovely scenery in Thedas. No doubt it's real purpose was to show off their new game engine but what it said was "this is what you are trying to save". It is worth saving, even if you don't think much to the intelligent creatures at the top of the food chain.
What has me at a loss is this apparent "choice" between "redeeming" Solas and "killing" him. I prefer the notion, whatever it takes to stop him - I seriously don't see how you're going to convince him not to go ahead with his plan but I'd give it a go. However, should the opportunity arise to stop him some other way, I'm not going to pass it up simply because I said I was going to redeem him. He's already made it clear that killing him may well not be a permanent solution, simply leave the possibility that like Mythal he will be back, so that doesn't seem like the answer. However, proving him wrong doesn't necessarily imply I give a s**t about whether he is redeemed or not. I might sympathise with him over the past but that doesn't excuse him trying to take away our future.