well he does sadly say '' Returning of my people means the end of yours '' I highly doubt that they wanted to make so many mysteries to that and made it pretty clear whats going to happen next. Now I am not saying everyone will die but most I would assume.. how? I don't know :S
I was correct about his orb giving theory so I feel like if the writers were pretty logical in that one then I would think they will go logical here as well. Trust me I also want to hope more XD and there is always a POSSIBILITY XD but I just highly think it's going to destroy many people some how 
"End of yours", "your world must die", etc.
Like his world died.
It could be metaphorical, he said he destroyed the world, but Thedas still exists, dwarves still exist, elves still exist.
I'm 100% certain the Veil will come down in the next game (there are 3 different prophecies foreseeing that and several prominent characters speaking about an upcoming change), I think that the sundering of Veil will not kill everybody because then Thedas will just end.
When he spoke about his previous plan to rip down the Veil via his orb he said that while the world burned in raw chaos, he would've restored the elvhen, I think chaos = spirits, demons, people getting magic, mages growing suddenly stronger, titans awakening, etc.
If having the Fade merge with the unchanging world will cause anybody to die, Solas will stop being grey, it will change him from a compelling antagonist to a full on evil villain. Moreover, (in my eyes) it isn't as interesting if there is nothing to be gained from the removal of the Veil, it takes from the story.