Adding to that, imagine how old Solas must be. I think about his character, and I feel bad for how much he has put himself through. I see it as high time for him to die, and Lavellan dying with him would assure that his greatest fear is never realized: he doesn't die alone.
And then their spirits are happy in the afterlife or whatever (in the fade, idk) and they can be together for eternity.
Solas may be very old, but to me it's not poetic that Lavellan, who is comparatively quite young (even if she's 80, that's young compared to Solas) and who could achieve immortality if the Veil is removed, should die with him. And "dying alone" does not necessarily mean someone else must die with you. It only means that you don't have to die as the last of your kind, or die with no one by your side. I just don't see anything beautiful in them both dying. There's no redemption for Solas, and Lavellan effectively gets punished for doing the right thing (trying to stop him).
We don't know precisely what the afterlife in Thedas is comprised of, either, so dying together doesn't mean a happy ending in the afterlife. In fact resurrection appears to be a thing. Spirits and souls may be the same thing, but a spirit can be destroyed during its rebirth, or it may simply not be at all the same as it was. In that respect, death (and therefore forgetting their lives and connection to one another) for both of them is probably the worst and saddest thing that could happen. Spirits are also mostly asexual, so no "lovers in the Fade," unless they were becoming mortal/preparing to embody again.
Again, I would prefer if only one of them died, or even better if neither of them died. But I repeat, Lavellan (or even a friended Inquisitor) dying to redeem Solas may be a thing that we see. I could see her intervening with a spell or something and winding up killed as a result, dying in Solas's arms. He stops with his plan and then proceeds to live his life as he feels she would have wanted. Alternatively, the Inquisitor decides that killing Solas is the only way to stop him, and if romanced, Solas ends up dying in her arms, perhaps with a final apology. Lavellan then decides what to do with her own remaining life.
I prefer that they both live, though. It doesn't necessarily have to be a sweet, sappy, happy ending. In fact it probably won't be, because more than likely Solas will come very close to achieving his goal. And, if the in-game prophesies are correct, the Veil will come down even though we do stop him. There might be a suspicion there that he carried the plan out anyway. There will almost certainly be a lot of pain between them because even though Lavellan may love Solas, he has done and said a lot of things to hurt her. There would be a lot of healing needed for both of them. Maybe Cole could help. Maybe not.