Remember how Solas talked with Cassandra about the Tranquil? How he said that the magic and connection with the Fade as well as control of emotions is like a muscle, and it atrophies without use? That restoring the Tranquil to previous state would likely kill them?
And that's just Tranquil who were sundered out of the Fade for some time, but not their entire life, while there are entire GENERATIONS of people (including elves) who have never had a strong connection to it. I think when the Veil drops, most people will experience what might be 'a magical overload' - the mostly unconscious connection of the Fade they have, with the rest being blocked by the Veil, it's too little to give them a protection before getting in contact with the raw Fade. Just like demons are unable to comprehend the real world, hence they appear monstrous and deformed, most people would likely not be able to comprehend the shifting nature of the Fade and their bodies will disintegrate.
Of course, there might be something else as well.
If corypheus hadn't meddled: "I would have entered the fade, using the mark you now bear. Then I would have torn down the veil. As this world burned in the raw chaos, I would have restored the world of my time... the time of the elves."
Notice that he didn't say that he'd restore the elvhen civilization. He said that he'd restore the world 'of his time' and 'of the elves', but not necessarily the elves themselves.
Most damning of all: "We aren't people to you?" "Not at first. You showed me that I was wrong... again. That does not make what must come next any easier."
Why would he be working so hard to save people he didn't consider people? Pre-corypheus, clearly he wasn't doing this for the benefit of the modern elves. The entire point of that exchange was him telling you that DESPITE starting to see modern people as full people, he intends to go through with his plan anyway.
Um, how do you know he wasn't doing anything for modern elves
? He said that he awoke a year before Cory and yet before that he seemed to have his network of agents, working towards his plan. He also said in base DAI that he tried to give elves and Dalish some advice, but was mostly rejected. Therefore I'm not sure why claim he's doing nothing, even if at that time he kept everyone alive in Thedas at a distance.
He also answered Inky why he's doing this for people he may still not view as his, even if they're about to die - to give the world some comfort and peace before the inevitable end. A calm before the storm.
I also assume he also has pretty pragmatic motives - war breeds demons and bad emotions, meaning more bad things happening to the Fade. The calmer things are, especially after calamity that was the Breach, the better for both worlds.
"Why bother disrupting the Qunari plot, if you're going to destroy the world regardless?" "I take no joy in what I must do. Until that day comes, I would see [people] free of the Qun. If they must die, I would rather they die in comfort."
...I don't know how he could make this clearer. EVERYBODY is going to die.
I think he was pretty clear about that
Question is what that 'death' means. Death of everything physical? I mean, it's pretty obvious at this point that everybody alive in this world begins as a spirit/has a spirit. It may yet be that what Solas plans is stripping everyone of their previous shapes and make them all like his people were before. Hence Sandal's prophecy - "everyone will be just like they were'. Which likely means spirit-like beings with shifting, half-physical forms.