Right up there with "I take no joy in this, but the return of my people means the end of yours". Pretty straight forward.
Again, I don't understand what has you guys so frightened. If it turned that it's all a big misunderstanding and his plan never represented a real threat, then there would be no conflict.
DA4's hero suddenly finds out that they don't really need to stop Solas.
You're being selective - you seem to only accept definitive answers, while ignoring those which imply that he's not certain what's to come. This is not exactly honest.
Also - I'm not really sure why you're making an assumption that people are frightened: that he may do something that pushes him past moral event horizon maybe, but one does not have to plot the utter end the world to do that. Look at Corypheus after all - his plan wasn't to 'destroy the world', it was in fact 'to give the world a god and nation it deserves' and give mortals comfort that someone 'up there' is looking out for them.
In theory this is vastly preferable, just from description alone, to end of the world. But we do know what hid behind that description - hubris, madness, elitism, utter disregard to what's sacrificed and stooping to most sinister of methods to manipulate or conquer people and nations, and that's just aside of the possibility that his insane plan will bury everyone. The HOW is therefore as important - if not more important - than the WHAT of a given plan.
Also - given that Corypheus was actually sort of a red herring and during Inquisition we basically built power that is able to withstand what's to come after this threat (and perhaps make Solas start doubt his plan if we gain his respect) there's no way of telling if this is the way DA4 will take (I mean aside from the fact that Solas is already a vastly different antagonist compared to Cory).
Will there be just one antagonist? Will there be more? Solas is pretty obviously terrified of the Blight and we know that there are other threats stirring in the background. So the conflict may not be as straightforwards as redeem/kill Solas - there are other routes the story may take. Is killing him the right path to go, given the shaky fate of elves/magic/Fade if we do, OR that we may need him to battle the real threat? Will we maybe have to side with some sort of unknown force to actually kill him? And what if it turns out that it's Inquisitor that is the threat at the end of it all? I mean, DA3 hasn't yet answered the question of whether Inquisitor will save the world or destroy it (Ellawynn has also provided few other excellent possibilities for future conflict).
There are so many possibilities with current set up, I'm surprised you seem to be expecting they'd stick to the simplest one. Especially given the fact that Trespasser doesn't just paint Solas as a straightforward threat, but offers us an option to redeem him.
These are strange parameters where the only acceptable conclusion is that this is a morally squeaky clean salvation for all, with shelther homes for all modern elves and nobody gets hurt. Or some people will, but mainly to rid the world of evil.
That's a rather white-ish gray color right there.
... Since when suggesting that the issue may be more complex than just 'destroying everything' makes everything "morally squeaky clean"?
Like... wut? Where do you even get "salvation for all" or "nobody gets hurt"? This is nothing more than a strawman, and a pretty annoying one.