Are you really arguing that harming the current inhabitants of Thedas *isn't* bad? I doubt that you are. (I hope.)
Ancient elves (that are still alive) aren't less deserving of life than modern people, but they aren't deserving of it at the expense of modern people, either. There is a current, possible solution where both groups get to live. Ancient elves will age and die like everyone else if exposed to the current world for too long, but they'd still get to live, and do so without it being at the expense of others. Solas is the one deciding one group (and assuaging his guilt) is more important than another group, as well as one standard of living for that one group being the only acceptable standard of living.
Forcing one group to just accept mortality and live a few years isn't really a solution - merely the world waiting for the 'problem' to literally die away. It's not really better than continuous diminishing of modern elves or dwarves: it's just slowly waiting for them to excuse themselves from the living world (and oftentimes helping them with it); a notion I find hard to accept, even if preferable to death of countless people in favor of one group.
But that group doesn't appear to be specifically the remnants of ancient elves. And we don't really know if ancient elves are truly mortal now - Abelas claims that perhaps only uthenera awaits for him, if fate is kind, suggesting that he himself is still immortal. Then there's the fact that Solas claims that "the Veil took everything from the elves, even themselves'. It didn't seem to take 'themselves' from ancient elves, at least judging from Sentinels, so he must have a broader group in mind.
Then there's also the fact that he claims that Veil has blocked most people's conscious connection to the Fade - the whole world is effectively semi-Tranquilized. That's actually what he gives us as a major reason for making a decision to lift the Veil - it might have saved the world from the Evanuris, but now it does injustice to all the living creatures and he thinks that restoring that connection is preferable to leaving it be, even if it effectively brings destruction to the world that emerged after the Veil was formed.
So it's not as simple as "saving a bunch of ancient elves".
And this is a speculation that yet awaits to be confirmed, but it may in fact be that the only reason he says that he'll restore the elves and their world is because most creatures will basically 'default' to what they were before - after all, aside from the Dwarves, it's lightly hinted at that both humans and Qunari may have had descended from the elves, who themselves descend from spirits and are close enough to it that they call spirits their brethren.