It will not be In Hushed Whispers. That world had a Veil. It was just severely weakened and harmed/harmful demons were pouring through rifts.
It will not be the Raw Fade, specifically the Nightmare Realm we visited. It will not have swarming demons and bizzaroland.
I want to make my opinion on that clear. I don't think it will be destruction everywhere in the way that Corypheus was a threat.
But it'll be big. The 'world' Thedosians exist in will be 'destroyed'. All institutions ended. All mortality put on the low rung of power. All non-mages put on the low rung of power more than ever. Both demons and non-demon spirits interact freely with Thedas.
It'd be too much for any of the existing forms of order on Thedas to bear, and this does indeed mean mass deaths and chaos as the new order forms/old order reforms.
Bringing down the Veil may also have a time component too, maybe even a bit like the Elder Scrolls 'Dragon Break', where the chronology gets 'crunched' and events become a new timeline that will effectively keep this Thedas around but also have it demolished by radical change and the return of Elvenan.
'World ending' and 'burn in the chaos' and 'likely die' could still actually not be as directly literal as it seems. But it'll still mean the end of this world, destruction, and death. With how Solas plans so far.
I think that at least in In Hushed Whispers we can imagine everyone dying within a few years, whereas the future of Solas' plan could instead mean an extended process of developments that change things so much that what we call the 'world of Thedas' no longer 'exists'. In the meantime, institutions like the Inquisition being strong could still be seen as a good thing, as it might hold out longer and provide refuge in the time being - both before the Veil going down (when it comes to facing the Qun) and after (when it comes to facing ..????).
What I think we need to be prepared for is that whatever comes, it won't be the same as the Breachy situation in DAI. Maybe there will be rifts involved, and a sort of thing reminiscent of the Breach, but we're NOT REVISITING THIS to such a degree ever again - it would make no sense narratively,.
This is about tearing down the Veil, not punching holes through it. It means bringing all of the Fade (the bad AND THE GOOD) in to interact more with Thedas again, not pooping out frenzied nightmared demons that can't even be spoken with. Sure there'd be a demonic threat everywhere, but its still not the same thing a DAI.
Solas' plan actually has a lot of pluses. Our problem is that the collateral damage of everything the writers had us prop up in the narrative of DAI - the institutions and society of (in this case Southern) Thedas. And on a larger scale.
I don't expect the regular Ferelden to stand any chance against the changes that the Veil going down would bring. At least with the Breach they could keep to themselves and be aware of the locations of the rifts and try to keep their distance, but there would be no/little 'distance' if the Veil is down. Its a state of the world that they wouldn't know at all. Even the early humans of the pre-Veil day were probably more prepared. I think we were exposed a lot to the Avvar in DAI for a reason; they'd be among the mundanes most prepared for better interaction with the Fade, unlike a city commoner, for example. Denerim could easily be razed to the chaos of even just a few days or weeks of this sort of new world, I imagine.
What rises up would be Solas' new society, one clearly involving some form of elves. But beyond that, is up to us, and maybe the writers if they haven't figured it out yet, to consider. I don't think Solas is planning to be a ruler though. In fact, I think he's prepared to be hated forever, even by those who end up thriving in the new world. He's terrified of, but ultimately prepared to die alone for this cause. How he'll die is another thing to figure out.